Watch it here. MacKay's basic argument was, "To believe the Earth is old or that evolution occurred, you'd have to believe that Carbon-14 (and all the other elements used for dating) worked the same in the past as they do in the present. Believing that takes just as much faith as believing the Bible!"
Um... No, it doesn't. Believing that things work exactly the same way as you've always observed in cases that you cannot observe is inductive reasoning. Creationists use it all the time. They haven't observed the future, but they know inductively that the sun will rise and that the laws of physics will continue as they always have. That isn't faith.
Of course, creationists will often argue that the only reason we can expect the world to work in a regular and orderly way is if it was created by a rational God. But if that's the case, then what right do they have to say that the world didn't always work the same way so that they can dismiss the results of scientific dating methods that show that the Earth is very old?
Besides, their solution still leaves open the question of why we should believe that God will go on behaving rationally in the future. Their solution solves nothing.
But all that isn't even the worst of their problems, because, as it turns out, there are ways to justify inductive reasoning that are completely logical. For example, I believe that it is more parsimonious (or simple) to postulate only one kind of cause for every kind of effect, and vice versa, and since simpler explanations are to be preferred (a view which I defend on a priori grounds in my book Atheism and Naturalism) then it follows it is reasonable to believe that the effects of the past that we observe today were created by causes that create similar effects in the present day. Think about it: If we observe Cause A creating Effect B, and we reason that it is simpler (and therefore more probable) to suppose that Effect B is only created by Cause A, then in cases where we observe Effect B without having the benefit of being around to witness Cause A occur, we can still be reasonably sure that Effect B was created by Cause A.
And that isn't the only justification for induction. Here's another one: Induction can be likened to the sample-taking done by scientists. Scientists will often take a very large sample of something, and then reason that what is true of the sample is probably true of the whole (of whatever they are sampling). For example, if I interview 10,000 random people, and 90% of them inform me that they will re-elect Barack Obama, I can be reasonably sure that this is true of the entire population of voters. It is logically possible that somehow my sample wasn't representative of the entire population. Maybe, out of the entire population, only 10% want Obama re-elected. But it is extremely improbable that my sample would be that far off the mark.
Likewise, when we reason inductively, we observe something so many times (equivalent to taking a sample) and we assume that that sample is probably representative of all cases of that kind of event.