If you're running 6.2 and above then just use the activation-key command.
If you're running 6.1 and below, you have to upgrade the code via the console, and after the code has been loaded it'll ask you if you want to enter in a new activation key.
To be honest, it's probbaly easiest to upgrade to 6.2 and above and then just use the command.
See http://www.cisco.com/warp/public/110/upgrade.shtml for details.
