import flash.display.StageDisplayState;

function toggleFullScreen():void
{
    if (stage.displayState == StageDisplayState.NORMAL) {
        stage.displayState=StageDisplayState.FULL_SCREEN;
    } else {
        stage.displayState=StageDisplayState.NORMAL;
    }
}

btn_fullsc.buttonMode = true;
btn_fullsc.addEventListener(MouseEvent.CLICK, buttonOnClick)

function buttonOnClick(event:MouseEvent):void
{
    toggleFullScreen();
}

 <param name="allowfullscreen" value="true" />