Mask isue

Hello here is my script so far ....I load outside moviclips in a container ..the problem apears when I want to mask that container with a let's say 300/300 mask (the mask also created in script ). Is this possible?, can anyone give me some pointers? Here is my script:

var cercu1:Object = new Object();
cercu1._x=7;
cercu1._y=200;
cercu1._height=100;
cercu1._width=100;

createEmptyMovieClip("mc_1",0);
mc_1.createEmptyMovieClip("container",99);
mc_1._x=7;
mc_1._y=200;
mc_1.setMask("cercu1");

x=0;
var mc_load:MovieClipLoader = new MovieClipLoader();
var mc:Array= new Array(3);
mc[0]= "/Headers/1/brokers.swf";
mc[1]= "/Headers/1/architect2.swf";
mc[2]= "/Headers/1/weddings3.swf";
mc_load.loadClip(mc[x],mc_1.container);
nextbtn.onRelease = function() {

if(x>=0 and x<2) {
x=x+1;
mc_load.unloadClip(mc_1.container);
mc_load.loadClip(mc[x],mc_1.container);
trace(x);
}
}
prevbtn.onRelease = function() {

if(x>=1 and x<=3) {
x=x-1;
mc_load.unloadClip(mc_1.container);
mc_load.loadClip(mc[x],mc_1.container);
trace(x);

}
}