Help! Flash Snow 3.0 dilemma!

Hi there, I’'m new to this forum and new to Flash for the most part. I’ve been relying on these tutorials to teach me the basics of ActionScript and such. Hereés the question:

I’m trying to create a particular effect using the Kirupa tutorial for Flash Snow 3.0.
The size of the stage I’m working with is 800x600, but I need the animation to only happen in the center of the stage, in an area approximately 400x400. I’ve tried masking to no effect and am wondering if anyone could give me some advice. I’m sure it’s something that I just need to enter in the script, but being a newbie, don’t really get it just yet.