Adding a microphone requires setting it as the default recording device in Windows.
If you have the default sound control panel for Windows (there are some obnoxious sound chips which insist on stomping on the default sound control panel), right-click on the speaker ICON in the system tray, select "Adjust Audio Properties", then select the "Audio" tab. Press the "Volumn" button in the "Sound Recording" section of the panel, and make sure the "Microphone" is selected (check box).