Well, there are games like Rage that work with the XBOX 360 USB controller natively without any drivers. It is not plug and play on OSX. Other games will require third party drivers to be downloaded from tattiebogle.net I think. But be careful, these third party drivers will interfere with games released that work natively with the 360 controller from Aspyr like Rage. Check Aspyr support for more info on that situation.
It can be all very confusing sometimes right?