var holder:MovieClip = this.createEmptyMovieClip("holder", 1);
var loader:MovieClipLoader = new MovieClipLoader();
var listener:Object = new Object();
listener.onLoadInit = function(target:MovieClip):Void {
	target._xscale = 50;
	target._yscale = 50;
};
loader.addListener(listener);
loader.loadClip("paper.swf", holder);

とりあえず動くようにtarget_mcに***を書いて・・・・などの確認を簡単に記載していただけると非常に助かります。

var holder:MovieClip = this.createEmptyMovieClip("holder", 1);
var loader:MovieClipLoader = new MovieClipLoader();
var listener:Object = new Object();
listener.onLoadInit = function(target:MovieClip):Void {
	target._xscale = 50;
	target._yscale = 50;
};
loader.addListener(listener);
loader.loadClip("paper.swf", holder);

onMouseDown = function():Void {
	if (flag = !flag) {
		holder._xscale = 100;
		holder._yscale = 100;
	} else {
		holder._xscale = 50;
		holder._yscale = 50;
	}
};

var holder:MovieClip = this.createEmptyMovieClip("holder", 1);
var loader:MovieClipLoader = new MovieClipLoader();
var listener:Object = new Object();
listener.onLoadInit = function(target:MovieClip):Void {
	//target._xscale = 50;
	//target._yscale = 50;
	trace(target.getIFlashPaper());
	onEnterFrame = function():Void {
		trace(target.getIFlashPaper());
		if (target.getIFlashPaper()) {
			target.getIFlashPaper().setSize(300, 200);
			delete onEnterFrame;
		}
	};
};
loader.addListener(listener);
loader.loadClip("paper.swf", holder);

// MovieClip 関連処理
var fp_mc:MovieClip = this.createEmptyMovieClip ("fp_mc", this.getNextHighestDepth ());
var fp_mcl:MovieClipLoader = new MovieClipLoader ();
var fp_mcl_lsn:Object = new Object ();
fp_mcl_lsn.onLoadInit = function (target_mc:MovieClip):Void  {
	trace (">> fp_mcl_lsn.onLoadInit() IN : [" + target_mc.getIFlashPaper () + "]");
	onEnterFrame = function ():Void {
		trace (">> onEnterFrame() IN : [" + target_mc.getIFlashPaper () + "]");
		if (target_mc.getIFlashPaper ()) {
			target_mc.getIFlashPaper ().setSize (300,200);
			trace ("## target._url=[" + target_mc._url + "]");
			delete onEnterFrame;
		}
	};
};
fp_mcl.addListener (fp_mcl_lsn);

// URL関連処理(キャッシュ対応）
this.addParam = function (p_loc_str:String):String  {
	trace (">> addParam() IN : p_loc_str=[" + p_loc_str + "]");
	
	// ネット接続時
	if( String(this._url).indexOf("http") != -1 ){
		var buf_date:Date = new Date ();
		var prm_str:String = "?" + String (buf_date.getTime ());
		p_loc_str += prm_str;
	}
	
	trace ("## p_loc_str=[" + p_loc_str + "]");
	return p_loc_str;
	
};

// ボタン押下時処理
type1_btn.onRelease = function () {
	fp_mcl.loadClip (addParam ("document.swf"),fp_mc);
};

type2_btn.onRelease = function () {
	fp_mcl.loadClip (addParam ("./doc/document.swf"),fp_mc);
};

type3_btn.onRelease = function () {
	fp_mcl.loadClip (addParam ("../document.swf"),fp_mc);
};

type4_btn.onRelease = function () {
	fp_mcl.loadClip (addParam ("../doc/document.swf"),fp_mc);
};

delete onEnterFrame;