How to use Occlusion culling

We were trying to optimize our apps and were unable to follow the instructions as per this link..

http://unity3d.com/support/documentation/Manual/Occlusion%20Culling.html

We are using MAC version and we were unable to select any object to Bake. So were unable to complete the process for the optimization. Could any one help us to come over this selection issue or the optimization process.

Chris

http://unity3d.com/support/resources/tutorials/video-occlusion-culling

a video tutorial that may be of some help.