You have to do it from command line when you run the server, not from inside the game. If youre using windows, make a shortcut to tremulous.exe, edit properties on the shortcut to say:
(your path here)/tremulous.exe +set dedicated 2
dedicated 0 = normal client game
dedicated 1 = dedicated server for LAN use
dedicated 2 = dedicated server for internet use, ie. send heartbeats to the master server
there are other command line options you might want to add, like:
+exec server.cfg
also do a search for tremded.exe on these forums, it's a better version if youre going to run a server online.