Masking (or just covering) the snow effect

Hey everyone,

I need help. I have successfully used the snow effect (v.3) from this site into my Flash movie, but I cannot manage to mask it.

I only want the snow visable through a single circle in the movie. I thought masking the effect with the shape would do, when it didn’t I simply tried to cover it partially with another layer and it still played as if it were on the top layer.

Am I forgetting something basic here? I don’t remember encountering this before. Anyway, any help would be appreciated as this is for a project due tomorrow. Yes I left it late.

Thanks!
Keith