I seem to have the same exact speakers (except that I'm only using the subwoofer and the two main speakers.)
Anyway, what I was going to say was that it has more then just the "green" plug to go into the PC. Sure enough, a lot of the plugs go into the seperate speakers, but there is a total of 3 plugs for the PC.
I've only got one plugged in, but my guess is that if you want all 5 speakers for a complete surround sound, you will need to plug the other two in as well.