Learning through tutorials - confused

I am very new to AS3 and attempting to learn the language through tutorials. I am trying to combine two tutorials to form a matching game with a start screen with interactive button the starts a game (which resides in a MC) then a game over screen with a play again button. The game worked until I added the start screen, now I can’t null or remove the start screen in order to run the game screen. Anyone want to take a look and give me a couple of hints.

I have not written anything for the game over/restart piece yet.

Thanks for any imput.