In that case, we can't help you - I am the only one of the developers who has a Mac at all, and here it's just a Radeon 9200 "powered" Mac mini. Here it works fine, but this thing can't do OpenGL shaders at all.
So, try the following:
Start the game. When you got the black screen, blindly type:
Shift-Escape
condump test.txt
quit
after that, the game should exit. Then, go to the .nexuiz folder and in there, into data. Post the file test.txt somewhere and give us a link so we can check which graphics extensions were detected. You can also try:
r_glsl 0
gl_vbo 0
apple_multithreadedgl 0
r_restart
to check if it has any effect.