A mi si me funciona bien, tienes el código así?
Código ActionScript :
package flashFullscreen_fla
{
import flash.display.*;
import flash.events.*;
dynamic public class MainTimeline extends MovieClip
{
public var fullscreen_btn:SimpleButton;
public function MainTimeline()
{
addFrameScript(0, frame1);
return;
}// end function
public function toggleFullscreen(param1:MouseEvent)
{
if (stage.displayState == "fullScreen")
{
stage.displayState = "normal";
}
else
{
stage.displayState = "fullScreen";
}// end else if
return;
}// end function
function frame1()
{
this.fullscreen_btn.addEventListener(MouseEvent.MOUSE_UP, toggleFullscreen);
stage.scaleMode = "exactFit";
return;
}// end function
}
}