AS3 Loader

// Define a variable to reference your exported-for-actionscript 
// library element. Just create an empty movieclip and set its 
// export-for-actionscript property appropriately
var header_mc:mc_holder = new mc_holder(); 
 
 
 
function init():void 
{
    /////////////////////////////////////////// BEGIN Add Header
    var loadHeader = new Loader(); // temporary variable to reference the Loader class
    addChild(header_mc); // add our holder
    header_mc.addChildAt(loadHeader,0); // (You could just use addChild)
    loadHeader.contentLoaderInfo.addEventListener(Event.COMPLETE, positionHeader);
    loadHeader.load(new URLRequest("loadee.swf"));
    /////////////////////////////////////////// END Add Header
}
 
function positionHeader(e):void
{
    header_mc.x = stage.stageWidth/2 - header_mc.width/2;
    header_mc.y = stage.stageHeight/2 - header_mc.height/2;
}
 
init(); // Make it so.