I'll get back with you on this after I get the manual downloaded and see what I can figure out. I'm on dial-up so this can take a while sometimes... may be tomorrow.
EDIT: actually, what do you know, the 7N400 Pro 2 seems to share the same manual, so here we go: look at page 22 of the manual and it shows the back panel and suggests some stuff. Looks like they want the sub plugged into the bottom jack and the L/R speakers into the middle jack, and they refer us on to page 79 for how to set the jacks to match the functions of the speakers.
Actually, look at page 78 instead. That covers 2-channel, and then I presume you just plug the subwoofer into the bottom jack. The photos aren't good enough for me to make out what those Windows panels say, but I'm guessing you can spec the jacks as you desire. If you don't have the little utility launcher they're illustrating, maybe uninstall the latest nForce drivers and feed it the drivers off the mobo CD to see if that gets it on the right track. Good luck!