You would need to jailbreak your iPT first, then run "installer" and look in the "games" section. Doom is in there. But there are no controls, so you're just watching the demo.
I have jailbroke it, and when I go under "Games" theres nothin there, I got NES and Mines and stuff but no "Doom" -- did they take it out in the latest update of installer? (I just did it yesterday)