Easy MouseDown question or hard Universal Access Question

Hi all,
I’ve been scouring the applescript documentation and the web, and for some reason I can’t figure out how to execute a simple Mouse Down test.
The simple question:
What is the syntax for a top level, non-application-specific mousedown event?

The longer version:
I’m trying to script an accessability feature for my brother. He wants to be able to use the Universal Access Zoom feature with a single click. I figure the best way to do this is by choosing “Zoom” from an applescript menu at the top of the screen, having click #1 zoom in to a predetermined ammount, and having click #2 zoom back out and end the script. super simple, except I can’t figure out how to detect a click. The following code produces the error “Expected ‘given’, ‘into’, ‘with’, or ‘without’ or other parameter, but instead got end of line”:

on mouse down
tell application “System Events”
keystroke “+” using [command down, option down}
end tell
end mouse down

Now I assume mouse down needs some argument, but I can’t find a table of what arguments it takes. I also don’t know that System Events is the way to go for this, nor how to do this the “right” way, by sending actual commands to the Universal Access preferences pane.

Any and all help is greatly appreciated. Thanks in advance,
David Glicksman