Download mumble here: http://mumble.sourceforge.net/

Install it, then go in the options and change the transmission to 'Push To Talk'. (you may need to tick the 'advanced' box at the bottom).

After that, go into the shortcuts dialog and assign a key to 'Push-to-Talk'.

Hit apply and go to Server -> Connect and select 'add new'.

Add this info:
Label: Negromumble Idle
Address: idle.negroserver.com
Port: 64738
Username: <your username>

Hit 'Ok' and connect to the server.