How do I record on a 13" MBP?
Physical location of the jack:


The audio-in jack on a 13" MBP is a 3.5mm combination TRS port. 

It will sense standard analog or fiber optic connectors. It also works with TRS jacks for stereo audio out as well as a TRRS jack which allows microphone input as well as control from iPhone/iPod compatible remotes. 

It can work as a headphone out, line-in, or digital audio in/out depending what's plugged into it. It can also accept an iPhone-compatible headset with mic and use the mic (and remote functions for music). Some non-amplified microphone input requires further hardware to properly deliver the signal. According to the specs it doesn't, but it seems like if it takes an iPhone headset mic, the signal will be delivered, but in a less usable form.

A popular USB connector like the Griffin iMic works well with more diverse input sources and contains an external D/A converter. The iMic support FAQ discusses the many aspects of sound capture and is a great reference on the topic.

This instructables guide on making a combination headphone+mic adapter might also be of use.