Problems with optical on Macbook Pro Santarosa 15"

Hello, hopefully someone here is able to lend a helping hand in this issue iv had for quite some time now...

On my Macbook Pro, i have a 3.5mm adapter for a optical cable connected, which is then connected to a Logitech Z-5500 Sound system.

My issue is i am unable to get proper 5.1 sound coming from this setup no matter what i try...

Even when playing dvd's (Quantum of solace) i am unable to get 5.1

Does anyone know what i can do to solve this and finally get 5.1 sound coming through my system?