The flat-panel iMac is sleek, streamlined and thoroughly confusing if you've just switched over from a more traditional, bulkier tower-and-monitor setup. There are no obvious ports or jacks for headphones, printers or other peripherals when you look at the computer. The iMac indeed has all of these ports; however, they are located on the back of the computer.
Turn the iMac around so you are looking at the back, or move around the desk or table so you can see the back. You'll see a row of differently shaped ports toward the lower edge.
Read the symbols above the ports. One of the round ports has a graphic that looks like a small pair of headphones. Within the row of ports, you'll see four vertical USB slots; these will have the USB symbol above them, which looks like a three-pronged pitchfork with uneven tines.
Plug your headphones into the appropriate port. The shape of the plug will make it obvious which type of port you need to use. Remember to turn the USB plug so that the white portion inside the port fits in the empty portion of the plug.