So, short explanation typed from bed (even though you, Masterpiece seem to have figured it out already)
You download Mumble from
http://mumble.sf.net and install it. Run it. Then a welcoming assistant will appear. You will be asked to set up a certificate (that is what the server will use to identify you), just click next to do so. Yeu will also be asked to configure your audio. Basically, select the right source, if you want to use voice activation adjust the two toggle volumes (at which level to start sending your voice, and at which to stop again), if you want to use push-to-talk (walkie talkie mode) you have to set a push to talk key (the button you have to keep pressed while talking). (I would advise people without headsets to use PTT, as voice activation can react badly on notebook microphones, with the result that everybody can hear you typing, a buzzing, and harddrive access sounds)
Then you're all set to join the server.
As Label you can choose whatever you like. The address is mumble.halfur.de, the port is the default one. Type in a username and connect.
Your name will appear in a list on the right side. There you can see three channels (Chatter, Comic and Relate for now), which you can join by doubleclicking on them (only one at a time). When you speak everybody in the channel will hear. While you're sending the lips icon next to your name will become red, the same applies to other people talking, so you can see whose voice you're just hearing.
Please right click on your name and click "register". That will connect your username to your certificate. Then only your program can use this username. Only drawback: if you want to log in from a different computer you need to take your certificate with you (you can export and import certificates with the "certificate manager" in the configuration menu.).