within CS2, click on the window tab at the top (alt W in for windows OS)
ensure options in ticked in this window. (for a short cut key: Alt W O )
On the top row now, you should see options. Select your magnifying glass and you will see option on the top left of the page... including a + and a - whichever is toggled is the one that the glass will perform.
Alternatively, if have the magnifying glass on, and it is set to zoom in, pressing and holding ALT whilst clicking will reverse the direction of the zoom