I hope you mean you've _bought_ it. Else I'd say you illegally downloaded a bad copy. (And we don't want to help with those here. See board rules.) I'd ask the company selling it for support first.
One idea that springs to mind, since I've seen applications doing just that because of it: Is your harddrive formatted in the newest case sensitive HFS+ version? Some apps have problems with that. (Adobe's apps for example.)
I would assume the video card might be the culprit. The MacBook uses the Intel GMA graphics. I don't know what the minimum video chipset is in order to run DOOM 3. Check id software's site for details.