Great news if you are running Snow Leopard because SL is now natively support Sony PS3 controller. Which means that if you had enabled the Bluetooth on your Mac OS X and press the PS logo on your PS3 controller, the OS will be able to detect it correctly. Previous OS build requires user to install a custom small-file sized driver in order for it to be detected.
Now after you had paired it with your OS, another problem exist. How to map the key? Unlike windows which has a Game Controller panel which enable user to calibrate and confirm the controller had been detected and mapped correctly, there is no way to do it on Mac OS.
There are several third party software available but I would highly recommend GameCompanion, which is available on Mac App Store here.
In short, GameCompanion enable you to map your PS3 controller button to a certain keystroke. Now you can use the controller and test it on your favorite emulator.