Collision between bullet and monster

1

Guys, i just made a 2d shooting game lately. I got a question, how do you make a collision between the bullet and the monters? Here’s my code:

 enemy_mc.onEnterFrame = function () 
    { 
        if (bullet_mc.hitTest(this._x, this._y, true)) 
        { 
            this.removeMovieClip(); 
            hit.text = "true"; 
        } // end if 
    };

But it doesn’t seem to destroy/remove the monster when it hits or even say ‘true’ on the text on the upper left corner.
Here’s my game:
http://www.swfcabin.com/open/1294787100
My Flash: CS3