In one of the menus you can select 2 channel, 6 channel, 3d - you need to make sure output is set to 6 channels
Either the "audio" or "sound mode" button on the remote toggles between 2 and 5.1 output. (can't remember which..)
Suggest reading the manual if you have one...
If non of the above helps,. I'll gig out my DSOTM and walk you through my settings.