Accessing a movieclip of dynamically loaded swf file

in (swfA.fla) I have this code in frame1

var module1:Loader = new Loader();
var admin = new MovieClip();
var moduleURL:URLRequest = new URLRequest(“swfB.swf”);

in (swfB.fla) I have this code in frame 1

box.visible=false; (and a movieclip instance on the stage, called box)

My question is how do I get swfA to access swfB and set box.visible=true;

this: (in swfA);
(my thought here is that ‘admin’ contains swfB.swf?) - this would seem very simple… but it…

…does not work and gives me this [seriously unhelpful] error:

TypeError: Error #1010: A term is undefined and has no properties.
at swfA_fla::MainTimeline/frame1()

Any help out there would be awesome!