How to connect Digital Optical Audio output iMac with Receiver Input

I have connected iMac with onkyo hts-3300 home theater system receiver through optical audio TosLink Cable.( http://www.amazon.com/Digital-Optical-Audio-TosLink-Cable/dp/B001ZU3G2I/ref=sr12?ie=UTF8&qid=1292865309&sr=8-2 ). Unfortunately there is no sound coming out from the Home theater system. I have verified the Receiver optical input is working fine by connecting with my TV optical audio digital out put and the it's working great. The only difference between TV and iMac is, on my TV, it has optical audio digital output and I can use this cable ( http://www.amazon.com/Tripp-Lite-A102-02M-Digital-connectors/dp/B0002WPREU/ref=s r11?s=electronics&ie=UTF8&qid=1297320792&sr=1-1) to make it work. I can also verified my optical audio TosLink Cable working because I can see the red light coming out from the cable. So the only problem could be from iMac. I doubt I need to change the additional setting to configure iMac to use external optical audio setting instead of the buit in audio setting. Hoe can I able to do it? or my iMac(IMAC 21.5"/3.06/2X2GB/500GB/9400M-ITP
MB950ZP/A) doesn't support optical digital audio out put? I have follow this thread http://discussions.apple.com/thread.jspa?messageID=12769423&#12769423 and to my surprise which doesn't work out for me. I doubt that i need to change some setting, but when I got to setting>> sound I cannot change any setting. I was quite depressed aldy. Can someone answer my question as i m a newbie here. Thanks a lot 🙂