I dont have Garage band but I will try and help best I can ,
have you selected your midi device in Garage band ?
Also you have to select in AD what module you are using, in your case a TD-6. You can access that by going to the beats page tab > click start up (Bottom Right side ) select Roland > TD-6.
You will want to save that as your default startup.
Roland TD-12 Ludwig A to E Drum Kit
Windows 7 64 bit
M-Audio Audiophile 192 PCI Interface