There are two separate categories in the search for meaning in life, one says that meaning is only gotten from an external source, that is God. Meaning in life cannot be found in doing or in gaining anything. In this view, if there is no god then there is no meaning to life. In opposition to this idea Prof McGinn tries to push the argument into an infinite regression. If meaning can only be gotten from an outside source, then who/what gives god meaning, another god? Obviously, that leads to infinite regression because the next question is obviously who gave that second god meaning? (I was about to type a response but I'll save it for later.) The argument might be that there is an internal source of meaning that god has, and the answer that Prof McGinn offers is that mankind also has internal sources of meaning in life so that there's no need for a deity to confer upon us meaning. On the internal sources for meaning in life there are three different ideas: hedonism, virtue, and philosophic pursuit.
Hedonism is quite simple, pursue that which makes oneself happy. Food, drink, sex, nice stuff, knowledge, money, etc. though there seems to be different levels of desires. Like the base/natural desires of food, drink, sex etc. as compared to the higher desires knowledge, understanding, etc. The hedonist sees meaning in these types of things. One has meaning in life if one gains things and does or is able to do whatever one wants all the time. Prof McGinn points out, and I'm sure you also see that this is an incredibly selfish view. It cannot by itself provide for a meaningful life because there's something missing. This kind of view leads to an evil life bent on only seeking one's own desires and has no problem squashing anyone that gets in the way.
Which then leads to the life of virtue. This idea says that the meaning of life is in virtuous actions. It's no doubt that virtue is a good thing, that's an obvious tautology. But, is the meaning for life found in merely being virtuous? This view is a pendulum swing on the complete opposite side from hedonism. the meaning of life is in serving everyone else and putting down oneself. This seems like a decent way to find meaning but also ends up oddly empty because if you spend your whole life seeking to serve others you'll never enjoy life yourself.
While those first two ways to seek meaning in life are obvious to many people, this last one is less familiar except to those that have studied philosophy. It comes from Plato, that the highest meaning in life is the pursuit of and love of knowledge, and that the highest pursuit of knowledge is philosophy. This type of meaning is found in seeking and loving knowledge. Those this too has its problems, because it seeks meaning at the abandonment of living life. This kind of meaning in life doesn't look at life itself just at the pursuit of knowledge.
Prof McGinn's answer to the problems of each aspect is that one must have a balance in life of the three pursuits. One should pursue pleasure but not at the expense of knowledge and virtue. One should pursue virtue but not at the expense of the other two. And, when the different pursuits are in conflict there is no wrong answer. Everyone, has meaning this way. Some are more virtuous than others, some more hedonistic, and some more philosophical, but everyone has meaning.
By way of rebuttal I'd like to point out a few things... First, making the other side fall into infinite regress is not a valid argument. He's misrepresenting the external source of value as only contingent on the existence of an outside source for meaning. Meaning that is imbued upon creation is a natural side effect of being made by a loving creator. It doesn't require the creator per se, it's just a side effect. Just as God is the source of all creation, He's the source of all meaning otherwise meaning if you try to give yourself meaning it's hollow worthless meaning. No matter how much a man insists he's important doesn't make it so. True meaning can only come from the outside. Parents give children some of their meaning but even without parents a child has meaning.
Secondly, all these examples cannot be universally applicable. Here's a few examples:
For hedonism, what about the person born into poverty or born as a refugee? That person, most likely, will never be able to enjoy any level of hedonistic pleasures. Is that person no longer valuable? With extrinsic value, yes he or she is still just as valuable as anyone else.
Take virtue, (this is the easiest one to deal with) in a humanist mindset there is no immutable standard for morality or virtue, so the goalposts are always moving. Does that mean that different people are valuable at different times? Also, how virtuous is virtuous? Is Mother Teresa or Ghandi the only ones that measure up? Is everyone else worthless? Where's the line? The rules are constantly subject to change and so is the line of who is virtuous enough to have meaning.
Now, philosophy. This is the most difficult one to answer but it's still subjective. Take the child born into a small primitive tribe in Papua New Guinea or someplace like that. That child will probably never have the chance to study philosophy or science or any other learning outside that little tribe. Does he or she have meaning? Arguably not, at least according to these standards.
I think I know how Prof McGinn would answer these questions though, so I'll give his presumed answer and then respond to that as well. I assume he would answer with some kind of reference to balance in life. Like the child born into poverty, as long as he/she did his/her best to enjoy the pleasures available that's meaning, and if meaning found in a balance between all of these pursuits, perhaps that child could find meaning in life by doing his or her best on the other sources of meaning. To the question of virtue he'd probably respond similarly, that as long as one does one's best in whatever situation they're in they'll find meaning in life. Same with the pursuit of philosophy, as long as you do your best with what you have you'll live a meaningful life.
To which I'd respond, then Adolf Hitler lived a full meaningful life. He did what he thought was right and he did it to the best of his ability. So much so that he led his country in a victorious conquest of most of Europe and through his allies a large portion of the world in general. He led the extermination of weaker people that didn't deserve to live (at least in his view). He was also well read and his book is still read by many to this day. So, by all accounts he lived a meaningful life. Basically, by these arguments everyone lives meaningful lives. Again, without an immutable standard of morality there's no such thing as a virtuous person. And, nothing can give itself meaning without being completely selfish. These anthropocentric sources of meaning and virtue ring hollow and even the most powerful rich and seemingly most meaningful lives are reduced to nought at the end. As Alexander the Great's final orders clearly showed, even one of the most powerful, wise, hedonistic (after the higher pleasures), wealthy, and philosophical rulers of the world ended his life empty and meaningless.