[MikroAcse]

Надо подождать, пока stage станет доступным:

package 
{
  import flash.display.*;
  import flash.events.Event;
  //
  public class Main extends MovieClip 
  {
    public function Main():void 
    { 
	  if(stage){
                  init();
             } else {
                  addEventListener(Event.ADDED_TO_STAGE,this.init);
             }
    }
    public function init(e:Event):void {
             var flashVars:Object = stage.loaderInfo.parameters;
	  var uid:String = flashVars.user_id ? flashVars.user_id : "mytestvalue";
	  p.pole_uid.text = uid;
    }
  }
}