here is the manual
http
/www.logitech.com/assets/33015/z506620-002649005326gsamr.pdf
each speaker should have its own rca style connector which plugs into the back of the subwoofer. the front left channel will have a large proprietary connector.
how it hooks up to the pc is with 3 male-to-male 3.5mm connectors. normally colored green, orange and black. these connect to the 3 jacks on the back of your pc (colored the same).
if you're missing one of the colored 3.5mm cables any male to male 3.5mm cable would work if its 3pole (2 ring + tip on connector). they are more common then dirt.