Hey everyone, I just purchased these speakers:

http://www.amazon.com/gp/product/B003VAK1FA

and I own a 15" Macbook Pro. Can anyone tell me what I need to be able to utilize the 5.1 surround sound that is possible with these speakers?

It ain't happening. Logitech makes 5.1 speaker systems that connect to Macs via the TOSLINK optical cable and work well. My Logitech® Z-680 5.1 speakers are such a system. Your new system is not. Apparently, it relies on a "5.1" sound card in PCs that have them. As best I can tell, your system works just fine as a regular stereo system. You get what you paid for. What you paid for is not a 5.1 speaker system for Macs.

According to this Apple Spec page (http://support.apple.com/kb/HT1596) you will need some TOS to Mini adapter like this (http://www.amazon.com/Recoton-Fiber-Optic-Toslink-Adapter/dp/B0002MQGRM/ref=cm_cr_pr_product_top). Then a TOS cable (http://www.monoprice.com/products/subdepartment.asp?c_id=102&cp_id=10229) to connect to the 5.1 speakers.

According to this Apple Spec page (http://support.apple.com/kb/HT1596) you will need some TOS to Mini adapter like this (http://www.amazon.com/Recoton-Fiber-Optic-Toslink-Adapter/dp/B0002MQGRM/ref=cm_cr_pr_product_top). Then a TOS cable (http://www.monoprice.com/products/subdepartment.asp?c_id=102&cp_id=10229) to connect to the 5.1 speakers.The adaptor is an optical connector. It will work on the OP's MacBook Pro because it's miniplug is a combination standard miniplug and mini TOSLINK. However, his speaker system does not support TOSLINK. He is SOL.

or cheap 5.1 or 7.1 USB soundcard .. off Ebay