I recently ordered an add-on for my game "Flight Simulator X", which
> is "PMDG 777". I have a problem with this game: the registration key
> is not valid anymore. I've tried to uninstall the game, I've tried
> many ways but it's still not valid.
> I am waiting for your answer
> Yours sincerely

You should submit a support ticket here: http://support.precisionmanuals.com/ for anything related to activation issues.