Odin, you first want to open up your Controllers List and find someone online. Once you see someone you want to monitor, you'll want to send a command to get their ATIS.
.atis OAK_CTR
Then you'll receive a message that looks something like this:
Voice URL: voice.radarcontact.me/OAK_CTR
(other stuff that isn't important)
You're going to want to take note of two things: the voice server (voice.radarcontact.me) and the voice channel (OAK_CTR).
Next you'll want to open your Comms Panel and fill out the following information in a new cell:
Position Name: (anything you want)
Frequency: (the frequency they are using)
Vox Server: voice.radarcontact.me
Vox Channel: OAK_CTR
Then click "Save Changes."
To start listening to the frequency, click "RX" and "HDST" next to the entry you just created and you should hear whoever you want to listen to.