Since iRacing has launched to Mac there has been issues for some people trying to install Force Feedback on Macs for there wheels.

Sadly to the best of my knowledge there isn't currently a way for Thrustmaster and Fanatec wheels to run with Force Feedback.

First you must install the Mac Kext utility found here: Latest Kext Utility 10.8 edition

Then once that is installed you must download this file and drag and drop it into the Kext utility when it is opened.

Then once install I think its necessary to reset your system for it to work properly.

Hope this helps.