Instances inside an Attached Movie

How do I manipulate an Instance inside an attachedMovie ?

For example, I create an createEmptyMovieClip and attach a Movie to it. The movie attached to it has an instance called mc_mask and I want to move that instance from outside that movie (from where I created the empty clip).

This dosn’t seem to be an issue with loading external SWF, with attached Movies it seems to be differen.t