When I run any of Nehes tutorials on my computer they all work except when I try to run them in fullscreen. It just shows the error message. Everything else works though. I have XCode not Project Builder and i just wanted to know if anyone knows whats wrong with it.

The Cocoa ports of NeHe's tutorials are based on the horrifically broken "Deep Cocoa" tutorials, and don't work in full-screen mode on many Macs.

http://onesadcookie.com/book describes how to create windowed and full-screen OpenGL contexts, and toggle between them, in a way that will work everywhere.