Your registration code should be located in the back of the Final Fantasy XI manual. If you have lost that entirely, then, there is nothing we can do to help you - since supplying you with a fresh code would be a violation of the ToS.

So, buy a new copy, is all I can say.. but before you jump the gun, look through your game manual, and make sure you noted down your POL ID code & pass aswell as checking for the FFXI Rego code.