External Speakers

I am trying to connect my Sony 5.1 ch. Surround Sound speakers to my iMac through the headphone jack/digital optical output on my iMac but I can't configure the speakers in "Audio/MIDI Setup" for Multi-Channel Audio. How do I connect my speakers correctly?

**Note: I normally use a MiniDisplay Port to HDMI cable to connect my speakers, and that works fine! But it was detecting an external display when I did that, which gave me some problems with software I was using so I wanted to try doing it through the optical output on my iMac.