The 13" MacBook Pros have only 1 audio jack, for both audio input and output, and you can chose between these 2 from the audio preferences.
If you have the Sound menu extra enabled, Option+clicking on it will let you chose what to use the audio port for (audio input or audio output).
[crarko adds: Handy shortcut.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20111106104816178