What do you mean you can't use a mic? Do your headphones and mic use the same cable? If so, then that's a 3-pole plug. The jack on this monitor is just a 2-pole stereo output jack so you can't connect a microphone through it. If your headset has 2 plugs then surely you can just plug that into your computer's mic jack?
maybe be a little more specific in your post as well. Model of both monitor and headset would be quite handy.
The mic normally uses a different port than the headphones. Why don't you just use the mic port on your PC/ laptop? Monitors are not known to have ports for mics.