iMac speaker input

Can't seem to find how to plug in a 3.5mm jack into the back of my 20" iMac running from my JBL Creatures...the input at the back of the iMac doesn't seem to take the full length of the plug...is there somewhere else...help please...

I have the same exact setup, except I have a 17" iMac.

It should be on the back of the computer, to the far left of all of the inputs/outputs.

There are two round plugpoints that are similar. One is marked "optical out".
Plug it in there. If it doesn't go in all the way, leave it and test the speakers. If they work, then it's OK, I've had that before.
If not, try a wee bit of force.

I had exactly the ame problem with my iMac Core Duo 20". My JBL Creature II speaker jack sticks out slightly but the speakers work just fine. I would not worry about it. It is obviously some kind of a design fault.