var prevNum:int = 1;

for (var i:int = 0; i< 10; i++) {
    this["Btn" + i].addEventListener(MouseEvent.CLICK , btnClick(i));
}

function btnClick(i_num:int) {
    return function (event:MouseEvent):void{
        var target : MovieClip = MovieClip( event.currentTarget );
        target.gotoAndStop("on");
		
	//ここのthisはMainTimelineではなくglobalを参照してましたのでMainTimelineを参照しました
		
        MovieClip(root)["Btn" + prevNum].gotoAndStop("off");
        prevNum = i_num;
    }
}