From the manual:
+"*Audio out port.* Connect external speakers, headphones (including iPhone), or digital audio equipment. You can specify audio analog line in functionality, to support line-level microphones and audio equipment, by selecting Audio In in the Sound pane of System Preferences."+
+"Choose Apple () > System Preferences, and open Sound preferences. Specify whether you want the port to be used for audio input
or audio output."+
This means you must choose whether you want the single audio port on the 13" MBP to be input or output. It mentions iPhone headphones, but those will work only as headphones, not as a combined mic/headset when connected to the MBP.
(As in, "Oh,
now I get it!")
Thinking on your comment, I see what you mean by 'an extra port' - the iMic has both in and out, meaning you'd actually have three counting the internal port on the MBP.
Looking back at your first post, you mention USB headphones. That would be fine as well - set the MBP's audio port to be used for input, and connect the USB headphones and use those for output. No 'extra' port that way, and USB headphones are cheaper than an iMic.
EDIT: The caveat there is that the audio in on the MBP (true for all MBPs, not just the 13") is a line-level input, not a microphone input. So, the MBP is expecting a line-level signal, meaning preamplified audio. If your guitar puts out a line-level signal or you're using a powered mic, fine. Else, you may
need the iMic, which can take either line-level or microphone (non-preamplified) input.