here are more details..it is not connected to a amp,the sound system has a 3.5 input,on the specs it says i can connect it to a computer,someone told me about a "Toslink" cable so i can connect them...is this right ??Wow. Your signature is not really filled with details.
You could give us more details on the receiving end, as the speakers are probably connected to an amplifier of some sorts.
It most likely has RCA stereo inputs, and the iMac has an 3.5mm analog stereo out. There are cables for that, they use TRS connectors, also called audio jack.
Also the iMac has an optical audio out (shared with the 3.5mm audio out jack), so if you have a digital in, you might use that.
If the speakers are active, they might have their own inputs, which are independent of an amplifier and the copper cable, so depending on the inputs those speakers provide, you could get a 3.5mm to 6.3mm TRS cable
EDIT: Your image doesn't state any input ports, but most consumer devices use the RCA plugs.
You need this:
im a customation and audio freak,hopefully it will work also?Wow, I really did not know about the optical sound. Something new everyday. Just looked into it. I'm hoping my 5.1 computer speakers have optical. I've been using them as 2.1's recently since I didn't feel there was a point to have all 5 speakers connected.
thanks for your help,ill send a linkIf you gave us the link to where you are getting these (I presume that you didn't draw them) we could likely make any further conclusions much quicker. But I still think that a standard 1/8" (3.5mm) cable will do the trick.