Parent SWF control from external SWF

I have one PARENT swf and one EXTERNAL swf.

on PARENT i have one movie clip called PARENT_MC.
on EXTERNAL swf i have one button with the following action:

myButton.onRelease = function() {
_level0.PARENT_MC._visible=0;
};

this does not work… please help… all i need to do is control a movie clip on parent swf from an external swf file.