This could be the common bug where you add the code, hit activate and airtool could ping the server twice. You get a positive response first time but when it pings second time, it throws an error. Ignore the error when it comes up. Simply restart AirTool and see if you have AirTool Premium at the bottom right of the main window. If you do, you have a premium option. If you want to double check, simply contact firstname.lastname@example.org with your username.