I know.. in my set. They came with a little converter that you can connect to the 3 wires.. which in turn gives you stereo input for gaming consoles.. etc. I just got a line adapter after that. The iMac only works in stereo.. but, it works quite nicely. Since the speakers have a "Matrix" setting which puts the sound in stereo to the front and rear speakers.
Are you on an iMac Core Duo? G5? G4? G3?
The iMac Core Duo has Optical outputs.. so if your speakers have an optical (Toslink) connection.. that's the easiest way.
However, if you don't. Picking up a USB soundcard which will provide the 3 outputs you need is about the only way to set it up for 5.1. Maybe some of the retailers can suggest some cards?