Making a movie clip load external .swf (HELP)

So I built a website which has movie clips for buttons and inside the movie clip is an invisible button so i can have my (rollover) events. Now i know how to call _parent and get frames from my main scene…but im looking to have a preloader load external .swf as done in this example… http://www.kirupa.com/developer/mx/preloader_transition.htm
The difference is they use normal buttons in the example and i cant figure out what code is needed to call from my invisible button inside the movie clip…heres the actual code on my button inside the movie clip.

buying_btn.onRelease = function() {
if (_root.section != “gallery.swf”) {
_root.section = “gallery.swf”;
_root.transition.gotoAndPlay(“closing”);
}
};

How would I make this work…Would i add a button handler to my code?..how do i do it?

any help would be greatly appreciated