Instructions
Click on the link provided in the References section in order to go to the download site for the PS3 controller driver. Click the "Version 0.01" link to begin downloading the driver.
Select the install file for the recently downloaded driver once it has downloaded to your computer. The install should begin automatically, and after it is over the drivers should be installed.
Connect the Mini-B end of your Mini-B-to-USB adapter cable to the Mini-B port on your PS3 controller.
Connect the USB end of the adapter cable to one of the ports on your iMac. The iMac should automatically detect the PS3 controller, and you will be able to use it.