Answer:
either your headset is defective or your settings is defective.
Explanation:
Your combined headphone/mic plug probably has four 'bands' including the tip.
Does your MacBook have one or two audio mini-jack inputs?
If it has one jack, you should be OK, but your headset mic could be defective. Have you tried a different combined mic/headset?
Older MacBooks have two separate jacks for audio input and headphones. 'Line in' audio expects an amplified audio source as input, such as a microphone attached to an external amplifier, or an iPod output through a stereo cable with a mini-plug on each end. Your headphone mic is probably not amplified.
This screen capture shows a (blue dot) peak audio indicator with a (mono) amplified mic as input.
If you have an unamplified mic, you likely won't get ANY sound indication from Line In, even tapping directly on the mic.
