There is a mic input port on the MBP right next to the earphone jack, except that it is "line level." This audio input port requires a line-level signal, so you would have to use a preamplifier with your microphone. If you search "microphone line level adapter" you'll find a bunch of them.
On other hand, since the use of USB microphones are becoming more common these days (which can be energized by the built-in USB currrent) it may be more uesful to get one of them instead of using the line level input for the older style equipment.
I have a MacBook Pro and to the left of my headphone jack is an SD card slot, to the right there is nothing. Next to the SD slot are 2 USB ports, then the video port, then the firewire port, then the network jack.
Why don't I have a line level input? I just bought this MBP a few months ago (Sept.2012) to replace an older MBP I bought in Nov.2009) which also does not have the line in jack you speak of.
My experience with USB mics is that there is a lag between audio and video. They can also cause video recording software to lock up or become jittery.
Which exact model?
13" mid 2012 has a single headphone/line in socket - should be an in/out selection in Sound prefs I expect (I don't have one of course)
15" mid 2012 should have seperate headphone and line in skts.
Retina models have line out only, except for provision for the Apple headset/mic combo.
I found that BOSE ear buds w/ mic do not work.
I did some searching and found that the iphone headphones w/ mic do work.
Once plugged in, the iphone headphones change the INPUT of the SOUND PREF to EXTERNAL MIC and, if I put my finger over the MBP mic, the mic input was working from my iphone headphones.
2009 MacBook Pro running 10.8.2