CS4 Calling A Frame In A MovieClip With A Button?

I've been reading over numerous tutorials and guides as well as the Language Reference for ActionScript 3.0 but cannot quite seem to find anything to learn what I need (which I guess is because everything I have read assumes you would already know how to do this sort of thing).On my stage I have one button (mybtn) and one MovieClip. Within the MovieClip I have numerous other MovieClips (the different parts of a character, leg, arm, head, etc).

One of those MovieClips, lets call it "upper_left_arm" (within the other MovieClip "character") is comprised of two Keyframes, frame 1 and frame 2. There is also a second layer with AS in it, which just has "stop()" in it so it doesn't endlessly play.When I click the button on my stage I want to change the playhead to frame 2 and then stop within the "upper_left_arm" MovieClip.I gather that I almost certainly need to use gotoAndStop.However, most information I have found concerning buttons, AS3 and the gotoAndStop command relate to "scenes" which as far as I can tell are nothing to do with what I want.

Code:
stop();mybtn.addEventListener(MouseEvent.MOUSE_DOWN, mouseDownHandler);
function mouseDownHandler(event:MouseEvent):void {
upper_left_arm.gotoAndStop(2);
}

The "(2)" is the part I think I have horribly wrong, I've tried various other things after "upper_left_arm.gotoAndStop" with no success. In fact, the MovieClip "upper_left_arm" now loops round merrily despite the "stop()" in the AS in that MovieClip itself .