If you mean for selection, then (in terms of Mac, might apply to Windows too):
1) Click on "File Menu"
2) Click on "User Preferences"
3) In the left-hand-most side of the Preferences window, you'll see (at the top), "Presets"... Now look lower and you should see the "Mouse:" section.
4) Further down that section, you'll see "Select With:"... This is where you choose which button (or side of the Magic Mouse) selects an Object. Right Click (or right side click on the Magic Mouse) is default.
Interesting: At the top of the "Mouse:" section, you'll see "Emulate 3 Button Mouse", which on the Mac (and Magic Mouse), turns a (Left Click + alt) into a Middle Click....
