I read the book a few years back. If you haven't you should really go out and get it now. It's phenomenal. Anyways, what they did was cut strips of flesh off the buttox and layer them on the white roof of the plane for a period of time (don't remember how long) to freeze dry them. They basically made human jerky.

I read the book too (the first one, "Alive"); actually I thought it was atrociously written despite the incredible story. Anyway, I seem to remember that some of the passengers wanted to cook the flesh (they made a fire using wooden crates that were in the plane; I don't know how they lit it) but the team doctor advised them that it would have more nutritional value if eaten raw.

Incidentally, I think the doctor was wrong; from what I know while cooking meat may destroy some nutrients it also makes the meat easier to digest, and thus effectively more nutrient-rich.