With mumble you need to connect to a server.
Menu Server->Connect
Then button "Add New"
Label: What you want this server to be displayed as on your computer, say "LinuxBasixMumble"
Address:mumble.openspeak.cc
Port:64747
UserName:ukBrian (or what you'd like to be know as)
Then you select that new server which should appear in favourites.. and hit connect.