Hee, hee. The manual for your surround system SUCKS.
Maybe I just didn't find the "good" version. I just realized that it's not a stand-alone receiver, but rather the electronics are built into the sub. I just modified the post above to reflect that.
Well, the instructions will be pretty simple based on the manual not giving much detail. When you start listening to the Xbox or the TV, try changing the sound mode on the sub unit. You can do this in at least two ways (refer to page 3 of the manual, which is page 4 of the PDF file):
1. Push the "CH MODE" button on the front of the sub.
2. Push the buttons on the remote labeled as "2.1", "3.1", "5.1", and "DSP".
The easiest way is almost surely using the remote. If you only hear sound from the front speakers, as you've said, push the "5.1" channel button. That might do it all on it's own. You should also experiment with pushing the "DSP" button. I haven't found yet what surround sound modes that system will do, but you'd probably want to be choosing some version of Pro-Logic because you'll be wanting to play two-channel audio through five speakers.
If you would, let us know if that works for you or if you have any other questions.