You have to make sure that you type the source exactly correct. Make sure you get the capital letters correct also. The iPhones autocorrect sometimes changes things as you type so pay attention to that also. http://www.marcogiorgini.com/iPhone/plist.xml
Once you get the source correct the games will be under 3D Games.
To check if you got the source right after you add it go to sources and it will be listed under MarcoGiorgini.com and titled "iPhone Homebrew Software".