You need to address the other movie clip properly. Right now your code is trying to go to and stop on the frame on the current movieclip that is labeled "ssbox1" You need to specify the address to the other movie clip like this:function play1(event:MouseEvent):void
If the other movieclip is inside another one you need to:
If you are unsure of how to address it, go to the movie clip you want to control and add the following actionscript:trace(this);
That should yield the address to the movieclip.