Why doesn't my headset microphone work?
On new Macbook Pros, there is no microphone input. What you have is a line-level input. Microphones that do not have integrated power and amplification (all cheap computer microphones/handsets) will not work when connected to the line-level input.

The connectors are exactly the same, but on the line input there is stereo input and ground, and on the microphone input there is mono input, voltage reference and ground. The input level is also different, with line level at around 1V and microphone level at 10-100mV.

To resolve the problem, you need to get one of the following:


a USB sound card which has a proper microphone input circuitry,
a powered, amplified microphone that can be used with line-level input, or 
a microphone preamplifier.


See also:


http://www.hobby-hour.com/electronics/computer_microphone.php
http://en.wikipedia.org/wiki/Line_level