BIG QUESTION! BIG QUESTION! mouseEvent

I create a box_mc on stage. I create another smallBox_mc in library.

my code:

package com… {

box_mc.addChild(smallBox_mc());

smallBox_mc.addEventListener(MouseEvent.MOUSE_DOWN, handleMouseDown);

private function handleMouseDown ($evt:MouseEvent):void {
do something();
}


}

Problem and error.
smallBox_mc undefine.

I name the instance name smallBox_mc is smallBox_mc

Any ideas?

thanks,

NB