ActionScript 3.0 :: Accessing Library Items In Loaded SWF From A SubClass?

how to access library items in the main SWF directly from a SubClass by doing this:
 
package{ import SubClass; public class Main extends MovieClip{public var mySubClass:SubClass = new SubClass;this.addChild(mySubClass); }//class }//package
 
package{ import Box; public class SubClass extends MovieClip { public function SubClass():void{var train:Box = new Box;this.addChild(box);}//SubClass constructor function }//class }//package

But how do I access library items of a loaded SWF in the same fashion? Importing is different from loading and I'm not sure how or where to piece instances together at runtime.