I have exported a movieClip within the library in the first frame of the movie
although I have not placed an instance of it on the stage. Inside this movie
clip on frame 1 is a function called addItem(). How do I call this function
from a button on the main timeline as I am having trouble getting this to work?

Cheers. Loop360.

you cannot call a function from a Library asset,
the clip must be fully instanciated on the stage
before you can access its function,