How to refer to mc in .fla from imported .as

AS3. i give up!! please point me in the right direction…

i have a html page of .fla’s that act like buttons and play external mp3s.
i will be making more buttons and fiddling with them in the future,
so i’m trying to create one Master.as that controls the basic functionality of each.

so, i’ve made a class as Master.as
which contains all the original code i had in the .fla.
then, i’ve successfully (i think) imported the package into Movie.fla.
Movie.fla has an instance “button_mc”.
but i can’t figure out how to refer to Movie.fla’s “button_mc” from within Master.as
consequently, of course, i get a slew of “1120: Access of undefined property button_mc”

i’ve tried root, parent, this, that and the other thing to no avail…