Those speakers are designed to be plugged into common PC motherboard surround sound connectors. Macs really don't have those connectors. I don't think these speakers do surround processing on their own given their price.
Assuming you want real surround sound, you have a couple options:
1. Find a USB sound device that has compatible 3.5mm outputs and plug that into your Mac. I don't know of any off-hand.
2. Find a surround-sound processor that can take optical in and has speaker outputs compatible with the 3.5mm jacks you have. Again, I don't know of any off-hand.
But even if you go either of these routes the surround sound is only going to work if you have Dolby-Digital encoded movies. iTunes isn't going to create 'virtual' sound-sound for you. An external surround-sound processor is needed to convert stereo music into virtual surround-sound.