5.1 Help

Hey,
I have a 5.1 Regent home entertainment system (HT-2004) and it is currently plugged into my computer through my M-AUDIO Fast Track Pro USB system. Each speaker is plugged directly into the subwoofer and then the subwoofer has left and right composite audio outputs (plugged into the Fast Track Pro). I have a Intel based iMac running leopard and it is not recognizing that I have 5.1 in the audio set up. Is 5.1 possible or are these just cheap speakers only capable of a left and right signal?
Please help!
thank you