Want to connect an external microphone to your MacBook Air? You can.

The 3.5 mm headphone jack is actually a combo jack, which is capable of supporting microphone input as well. But for microphone input you must use a TRRS jack (the ones with two black rings, such as on a phone hands free kit). The jack ouputs a headphone-level signal that can also be used to drive powered speakers, but not passive speakers.

Presumably this is also true of MacBook Pro.