World of Warcraft, as many other games, doesn't have native support for a Joystick or Gamepad.
The solution is to map axis and buttons to key strokes using a device mapping software. I've tried USB Overdrive and ControllerMate. These are my conclusions:
USB Overdrive: Easy to use but limited in functionality. The main drawback is that it takes ownership of the Magic Mouse and converts it into an ordinary 2 button mouse. The programmer seems working hard to fix this issue. Stay tuned.
ControllerMate: Very powerful, a bit daunting at first. It kept my Magic Mouse controlled by the Operating System. It can even create virtual devices mapped to the Joystick that can be used by any application.
My World of Warcraft Configuration:
Mac OS X Hints