Is it a 13" Mac Book Pro?
I have a 13" 2.26 GHz Intel Core 2 Duo Mac Book Pro.
It has a single audio port that can function for both input and output. In System Preferences>Sound>Input there is a drop down menu to choose to use the audio port for Input or Output.
Alternatively, I don't see why you couldn't use a USB microphone as well.
I'm guessing you have a 13" Late 2011 MacBook Pro. That has a single audio port which doubles as input and output. See page 24 and page 54 of the user's guide:
If you need to use an external microphone and also have audio output, you'll need a USB audio device such as the Griffin iMic.