I did some digging and found the problem and unfortunate solutions.
The macbook pro does not power the mic input and thus your mic is not being powered.
Solutions:
1.) Use the headset for listening and the internal built-in mic for talking.
2.) Purchase a USB Audio Controller which splits one of your USB ports to two connectors. One would be for your heaphone and the other for you mic in. This then requires a Y-Splitter from you mic headset to slit the signals apart.
Unfortunately those are your solutions. It seems like they want you to buy an accessory to be able to use mics such as yours which is a real shame. Perhaps they did not want to power the mic in to save power, but how hard is that anyway?
I feel your pain as many others have. The Griffin iMic is $25 and probably your cheapest solution.
Try this $20 solution:
http://www.amazon.com/Headset-Buddy-Adapter-Convert-Computer/dp/B0030E9FKO/ref=sr_1_9?ie=UTF8&qid=1314121444&sr=8-9
