This should be presented as an option when you are establishing a Quake server. I believe that earlier versions of Quake used 6666 as their default port.
If you can't find it, though, disconnect from the net, drop the firewall, open Network Utility and do a port-scan on your own machine (localhost) to see which ports are in use. Then, re-establish your firewall and open the port that Quake uses.