ActionScript Classes Problem

Okay so i created a button on the stage and made it a movie clip and called it’s instance name “play_btn” then i made a document class “main.as” and a second class “play button.as”

so what i want to do but can’t figure out how is to access the instance name in the play button.as file like this…

public class login_page extends MovieClip {

	public function login_page() {
               
              play_btn.addEventListener(MouseEvent.MOUSE_DOWN, onPlayDown);


	}
           
           public function onPlayDown(evt:MouseEvent):void {

             trace("Test");

           }


}

and then i want to call the function in the main.as… but the general problem is that i want to access the instance name “play_btn” in the “playbutton.as” file…

Sorry for my bad english hope someone can help me

Thanks:)