Why can't I access button from inside MC?

I am learning how to create menu systems for a game and have a few different “screens” (movieclips with buttons and such).

I am getting Undefined Property errors when I try to access a button from the class file of a screen.

Example: I have a MainMenu screen. This has its own class MainMenu.as.

Inside the library MainMenu movieclip, I have a button, instance named start_button.

But inside the MainMenu.as file, I try to do start_button.addEventListener(…) and it says undefined property.
I tried doing getChildByName(“start_button”), that won’t work either.

I am following a tutorial, which makes this even more frustrating.