How come my xbox live headset is not recognized as a mic by the game rockband?

I have read in several places that the xbox live headsets can be used as mics in the rockband games. Well i plugged it into the controller and restarted the xbox just like the manual says to do but when i try to play using a mic it says some thing like "please connect between one to three mics one for each vocalist" or something like that. I made sure to unmute the mic etc. What do i need to do to be sure the game and the xbox recognizes the headset?? BTW the controller is wireless since both USB slots are taken by guitars.

Thanks a ton!!