Meetn may not be able to see your microphone right away if you connect it after Meetn has been loaded.
First you should check the Menu > AV Setting (Audio Video Settings) and check the drop-down menu to see if your microphone is there. If it is, just click or tap on it to change over and you should start seeing your voice reflect in the test meter,
If you still don't see your mic, try completely closing the app or refreshing the webpage and rejoin the Meetn room. Now you should see your mic available.
If it's still not showing up, it's likely that your device hasn't properly initialized your microphone. Try disconnecting it and reconnecting the microphone to your device. Then restart / refresh Meetn. Now you should be all set!