I am trying to use my Creative HS 390 with my imac. The headphones work but I cannot get the microphone to work. How do I do this?

What Mac do you have, running what version of OS X?

If this Mac is an older Mac, many PC style headphone microphone headsets that use a small 1/8 style jack that plugs into the headphone jack will NOT works as Apple, usually, uses a non-standard, special length jack that makes using these type of audio headset mics a very difficult proposition.

There are very few of these style headset mics that work with Apple Macs and those that do or did will be hard to obtain now as they are, probably, out of production.

Depending on your OS X version, you will have better luck finding a USB connected headset mic or USB connected webcam for use with your older Mac.

Even with USB peripherals, you still need to make sure that it states the device is Mac compatible.

Sorry and Good Luck!

Just to add to MichelePM's response, older Apple products had a "PlainTalk" microphone jack, designed for Apple's PlainTalk microphones having a built-in pre-amp to boost the microphone's output to line level. The tip of the longer plug received the 5-volt power for the microphone's incorporated pre-amp. If you want to use any of a number of "PC" (non-USB) microphones, you'd need to find one of these discontinued "Griffin NE Mic" adapters. For the hobbyist, a PlainTalk microphone can be converted to a pre-amp only, for use with other non-amplified microphones. The modification (which is described on this page) involves removing the microphone element and installing an internal 3.5mm jack in the microphone.