Zaxwerks Proanimator and masks

Hello.

I am trying to make an animated logo, with letters getting stamped on a rectangle. I modeled it in illustrator and got the letters to animate like i want to, but now, i want to make an explosion-like effect when each letter gets stamped on the rectangle.
I am using AE cameras, not the proanimator one.

I tried to create a layer mask on the proanimator layer, but it doesn't
do anything to that layer, its just like its not there.

Any idea how can i mask each letter so that the explosion footage i have be seen from behind the letters?

Thank you,
Sebastian Plamadeala