external mic

I have an external microphone that i would like to use with my sgs2, but I cant get it to work...

It has 2 dark lines on it, and i have seen that the headsets have 3 but the therd must be audio and i dont have any just want mic-line in? I mean the headseats work fine so I must be able to use a microfone as well!

Does any body have an Idea?