ActionScript 3.0 :: Accessing Instance Name In Play Button?

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". 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. I tried to link it to the class but i get this error: 1120: Access of undefined property play_btn.