How can I use a non-amplified mic on a MBP?
You can't get good results with an unpowered microphone without a cable adapter or USB interface. The best bet is to purchase one of the many USB to audio interface dongles available. A favorite of mine is this one from Monoprice in California, which costs under $10. It includes a stereo headphone output and a mono mic input. It will also handle  powered microphones, such as the Audio-Technica ATR-3350 lavaliere mic. I have two of these mics, and they offer excellent value for money (about $20).