When you key the mic, if you are not getting the beep, then you need to make sure the mic is set as the primary recording device in the audio control panel.
If you are getting the beep, then make sure you have set the "V" channel box in the radio control bar to "101".
If you have the channel set to "101", and hear the beep, but no on appears to hear you, then set channel "V" to your game ID and see if you can hear yourself. If you cannot hear yourself, then it could be a firewall issue.