If you have a home LAN, - for example if you connect to a wireless router to connect to the internet - so long as both computers are connected to the router you should be able to connect from one to a bzflag server running on the other. Just type the IP address of the computer the server is running on into the server field in BZFlag.
If you can't connect in this way, you can create a quick ad-hoc wireless network quickly on the mac - assuming both the mac and the compaq have wireless cards. On the mac choose the airport menu, and select 'Create Network', then follow the onscreen instructions. Then connect your compaq to the wireless network you just created, by joining like you would a normal wireless network. Congratulations, you've connected your two computers.
Alternatively, if you want a physical wired connection, connect a cat5 ethernet crossover cable between both computers. This should connect the two computers without any further configuration.
To actually start the BZFlag server, there is a great deal of info on these forums, and on the bzflag wiki,
http://my.bzflag.org/w/Main_Page that you will find helpful. Just make sure you omit the '-public' option when you start the server. This option lists your server globally, which you probably don't want.
, so that I can play BZFlag with just two computers, one being controlled by my brother.
