Steelseries Artic 5 Headphones mic doesn't work on my iMac

My iMac is MacOs High Sierria version 10.13.1, mid 2011. I bought a Steelseries Headphones (the Artic 5), which is compatible with Mac. It uses USB cable. When I plug it in the iMac, and I went to setting and sound, the output part is showing that my headphones is plugged in, but in the input part, it doesn't show the option for my headphones mic, so I couldn't use the mic and I have to use the mic of the Mac, which is not too good.