addChild from .as file that isn't the document class

Ok I’ve been searching for so long I can’t remember when I started. I’m quite new to AS3, but am very well versed in AS2. As such, I can’t do many simple things in AS3 that take 1 second to do in AS2, so I would appreciate any help.

All I want to do is attach a movieclip to the stage via a .as file that ISNT the document class.

So for examples sake lets say I have
Main.swf - Main swf file with a movieclip called “PreloaderGUI”.
Main.as - The document class, this loads Preloader.as.
Preloader.as - This preloads the movie and attaches PreloaderGUI.

Can somebody please explain how to do this seemingly simple task? I have tried god knows how many different ways and I cannot for the life of me get ANY of them to work.

Thanks in advance to anybody who helps.

Brendan