Have you tried changing your rendering subsystem? If your rendering system is set to Direct3D9, change it to OpenGL (visc versa). To change your rendering system, to into C:\Feral-heart. Once you're in the main folder, you should see an "ogre.cfg" file. Click on it, open with Notepad, and look at the first two lines. If the first two lies are:
Render System=OpenGL Rendering Subsystem
[OpenGL Rendering Subsystem]
Then change it to...
Render System=Direct3D9 Rendering Subsystem
[Direct3D9 Rendering Subsystem]
Also, since you're running the game on a Mac computer, have you downloaded the Mac installation?