[PainKiller]

ну и сделайте все в одном классе:

package bk.file {
 
import flash.display.MovieClip;
import flash.events.MouseEvent;
 
	public class korpus extends MovieClip {
 
var myLoader:Loader;
		public function korpus()
		{
			buttonMode = true;
			addEventListener( MouseEvent.MOUSE_DOWN, down );
                        myLoader = new Loader ();
		        myLoader.x=50;
                        myLoader.y=50;
		}
		protected function down ( event:MouseEvent ) :void
		{
			parent.addChild ( this );
			startDrag();
			stage.addEventListener ( MouseEvent.MOUSE_UP, stageUp );
                       var myURL:URLRequest=new URLRequest("swf/korpus_buksa.swf");
	                myLoader.load(myURL);
	                addChild(myLoader);
		}
 
		protected function stageUp ( event:MouseEvent ) :void
		{
			stage.removeEventListener ( MouseEvent.MOUSE_UP, stageUp );
			stopDrag();
 
			if(dropTarget)
			{
				if( dropTarget.parent.name == "korz")
				{
					scaleX = scaleY = 0.5;
					alpha = 0.5;
					y = stage.stageHeight - height - 300;
					buttonMode = false;
					removeEventListener(MouseEvent.MOUSE_DOWN, down);
 
				}				
			}
		}
 
 
 
	}
}