Masking and coding

i’m using this code to load external movies

function loadEx(mc){
this.createEmptyMovieClip(“myMc”, this.getNextHighestDepth());
myMc._x = 190.3, myMc._y = 126;
myMc.loadMovie(mc);
}

loadEx(“home.swf”);

but i’m trying to have them masked inside a box. how does one go about this? i have objects in the movie flying in from the side and i don’t want to be able to see it outside the box.