Pon ese codigo en la primera frame con el loader.
Código:
stop();
this.addEventListener(Event.ENTER_FRAME, loading);
function loading(e:Event):void {
var total:Number = this.stage.loaderInfo.bytesTotal;
var loaded:Number = this.stage.loaderInfo.bytesLoaded;
bar_mc.scaleX = loaded/total;
loader_txt.text = Math.floor((loaded/total)*100)+ "%";
if (total == loaded) {
play();
this.removeEventListener(Event.ENTER_FRAME, loading);
}
}