The new MacBook Pros don't have a line in through the headphone port anymore. I know, stupid. But it's gone. It used to be two separate ports (my 2008 MacBook was) then it was a combo in/out, and with the retina models. It's just gone.
The only mic that will still work are ones that use a three pin connecter and have an in-line mic with the headphones. Such as the Ear Pods or many others. But there is no standard line in anymore. You can get a USB audio interface fairly cheaply that will work. But it is a shame that it's no longer included. I used to use it on my '08 all the time for simple stuff.
Illustrated: