First off you'll need to register the game to your email address. You can do this by going
here (log in with your forum details at the top right if need be) and entering your serial key from the game.
Once you are registered, in the popup that opens when you try to run the game at home, enter your email address from your forum account and the serial key, and try to continue. It will try and fail since you don't have internet, and offer you an option to use email. You'll need to copy and paste a subject line and a block of gibberish to notepad and save it to a floppy/CDR/USB drive, then take it to a net connected machine and email it off as directed. You'll then get a file in reply to place in the galciv2 folder on your home machine to let it run.