You want to put the switch into the "ON" position, set the BDP-83's speakers to "large", set the BDP-83's subwoofer to "on", and set the BDP-83's downmix to "5.1CH". Once you've done that, you can press the "6CH" button on the 950's remote to select the multichannel analog input.

You might also consider using a coaxial or optical digital audio connection for CD's and DVD's so you have access to the 950's surround processing, although the BDP-83's analog section is good enough that you may end up sticking with the 6CH input all the time for simplicity's sake.