Why isn't my new microphone working?
From http://www.modmic.com/pages/faq:


  For Macs you'll need a power source like an iMic to use the line-in, or an adapter like this to use the headset jack, or and adapter like this to use a USB port.


Unfortunately, I do not have the reputation to link what was referenced in the FAQ, you'll have to visit the FAQ yourself.