problem with movieclips

Hi....I have a small problem with movieclips. I have a movie clip, say A. In this movie clip, I am trying to create a mask effect with an image and a small square, which in turn is a movie clip, say B. What I am trying to do is when the movie starts, I want different copies of the movie clip B acting as masks on the image. How can I accomplish it? Any ideas??

I wrote a small ActionScript code for that but it doesn't work. I seriously doubt the way I refer to the movieclip B.

==================================
count = 1;
while (count<=10) {
duplicateMovieClip("_root.boxx", _this["boxx"+count], count);
_this["boxx"+count]._x = random(1000);
_this["boxx"+count]._y = random(100);
count = count+1;
}
==================================
I have the same code in two frames in the timeline of the movie clip A. Is anything wrong with this?

SJ.