Possible problems with my audio output jack for imac.

I used to have "Accent" speakers and they would only work if i plugged them into my headphone jack. I thought it was because they weren't mac based speakers and they worked fine thru the headphone jack so i thought nothing of it.
Now I've bought those little JBL speakers u can find on the apple store site and even THOSE don't work when i plug them into the speaker output jack. Only the headphone jack seems to work. There's no software and there seems to be no other way to inform my mac that i've hooked up an alternative speaker device. With a pc you can at least choose from a list when you've hooked up something new to get it to try and find something.
Am i missing something here, or is there only a special type of speaker that works with this imac thru it's speaker output jack?