Masking

I am having a little bit of trouble masking. I am doing something that is a lot like to sliding menu tutorial except that the bottom layer is something I have pasted on and then on top of that I have a bar that will scroll and on top of that I have the area I want to mask to bar onto. Because the bottom layer is a object and not just part of the background like in the tutorial I can’t get the mask to stay over it.

Is there a way to cut out a window in the bottom object that I pasted on and then mask over that?

I can’t get the mask to stay over it
??? Can you show me ?

pom 0]

nevermind, I figured it out, thanks though