Traditionally, mice on Macs have had just one button (including the iBook/Powerbook trackpads) – even through Mac OS (including OS X) can accept 2 or 3 button input without any problems.
Emacs on the other hand maps quite a few functions to the second and third chords of the mice. If you want to emulate a three button mouse in Emacs on OS X, a simple customization option will do the trick.
M-x customize-group and enter mac. Navigate to the “Mac Emulate Three Button Mouse” and select option 2 or option 3. Voila!
Note: This works for Carbon Emacs – the customization group is not available on the latest 23.1 Cocoa Emacs.