On the panel that smorodina pointed you to, click Options... Properties. Make sure your sound card is in the Mixer Device box at the top, then make sure Microphone is checked in the box below for both Playback and Recording. You probably want the mic to be muted on the Playback panel because otherwise it will play the mic noises straight back through your computer speakers either giving you feedback or just annoying you. Make sure it is not muted in the recording panel and it should be fine then.