attachMovie and setMask, help please

Hello, i am working on a piece of code, it does not seem very complicated but I am stuck, here is the deal:
I have a empty mc on top of another mc with a figure in it, using set mask I make the empty mc work as a mask over the figure, nothing shows, great. Now I want to have the user click on empty mc and attach a mc inside it to mask the figure and show a little piece of it. And it just does not happen.

here is the code:

empty_mc.onRelease=function(){
this.attachMovie(“ball”, “b”, 2);
}
empty2_mc.setMask(empty_mc);

Also I need to duplicate the ball mc , so every time the user clicks on empty mc a smal piece of the figure shows

Dont know if I am making myself clear, but I really need help with this

Thanks in advance