Hmmm... could be PEBKAC
1) Where are you plugging it in? Front or Rear?
2) What PC, what OS?
3) Are you sure your MIC is selected as input? Go to the Windows Mixer (you DO have windows, don't you?) -> Options -> Properties -> Recording, then select the recording source.
Maybe you have to select which MIC input to use, front or rear. You can do this in many ways, depending on sound chip type, OS type and installed driver. One option is with some sound utility that might be in your system tray. There you might be able to select.