I have the same issue as you, the mic i used on my MBP for skype doesn't work on my MP.
I read somewhere one this forum that the line in port does not supply any power out, unlike the MBP.
The best thing to do is buy a USB microphone or get a USB adaptor to supply the 3.5" jack with power.
Right now i'm using the Logitech Sphere camera for the microphone. I wish Logitech would bring out some mac drivers so i would use the tilt/pan!