Nomo is dead on.
Check out page 16 of the manual to get started on it. The note on there reads:
"If you’re using only one surround back speaker, connect it to the left (L) SURR BACK SPEAKERS terminals."
You can run the auto set-up to determine which speakers you have (as Nomo said), or you can do it manually. Check out pages 72 and 73 of the owner's manual for instructions on how to set it up for only one surround back channel.