Logitech speakers on iMac

After realising that the 5.1 speaker-system would not work with my iMac 27", I bought a 2.1 set: Logitech Z523 with two speakers and a subwoofer. Easy setup, it said... But: I have no sound... I connected the set to the headset-jack on the Mac.... help ?? what's wrong and how can I solve it...