Calling a function in a movieclip

I want to call a function that I define in a movieclip so that I can access the variables in the movieclip.

So, outside of the movieclip I want to do mc.myfunc(4, 5) and then the movieclip has a function called ‘mc’ that takes two arguments. How to do this? I have tried defining functions in the first frame of the mc but to no effect.

I’m using attachMovie so that may be why I can’t call myfunc, so how to get around it?