I have linked a PS3 controller to my mac via bluetooth and it is showing that it is linked. However when I try to use it in a game on steam I can't get it to work. I've tried reassigning the buttons in the options menu but it still doesn't work. Does anyone have any ideas on how to get it to work?

You need to install drivers for the controller as well, and I'm not sure if they are available for the Mac. There are unofficial drivers for Windows, though. Maybe for Macs too.