Accessing nested Movieclips from loaded SWF

I am having a newbie problem here. I am loading an external SWF with embedded movieclips (all clips are set to export in the original file). However, I can’t figure out how to access those embedded clips in my new, parent, flash file. Here is my code:

var container:MovieClip=new MovieClip()
var house_Loader:Loader = new Loader();

container.addChild(house_Loader);

var houseURL:URLRequest = new URLRequest("boxtest.swf");
house_Loader.load(houseURL);

addChild(container);

//PROBLEM CODE BELOW
house_Loader.box1.visible=true;

‘boxtest.swf’ is just 2 boxes with instance names ‘box1’ and ‘box2’. They are both set to export.

I think the issue lies in trying to get house_Loader to act like a movieclip, but I don’t know how else to do it.

Thanks so much for any help. This is my last big roadblock on this project.