NOTE: This is for the wired version of the Xbox 360 controller. It looks like you need to get an adapter since the controller doesn't use a standard wireless protocol like Bluetooth.
The Xbox 360 controller makes for a great controller for all sorts of games on the Mac, but sadly it isn't simply a plug-and-play situation anymore. Nowadays you need a custom driver installed to get it working, but thankfully there are some enthusiasts out there who are keeping this controller alive.
Simply go to this link to download the updated driver for your Mac. Like I said, this is developed by enthusiasts in their free time, but it works wonderfully for me.