Just store it in your preferences file, or in a file in ~/Library/Application Support. Yes, users will be able to send the preference file to another user, but it avoids inconveniencing your legitimate users. And, of course, you could encode the MAC address of the computer into the preference file somehow (not necessarily into the registration code, but into the preference file, so the user would never tell the difference unless he tried to copy the prefs to another machine) to discourage casual piracy this way. Be aware, though, that the user can still just give his regcode to another user...