This is really strange.
Looking at your site with Firefox Web Developer, and also with Firebug, it appears that the class (.categoryListBoxContents) that should control that element is in the stylesheet, but is not being applied to the element. Editing does nothing, and the margin already specified there is not in use. Adding !important or adding more specificity to the selector does nothing, either.
Looking at the active stylesheet.css with Firebug shows a different set of code (missing .categoryListBoxContents) than you get when selecting "edit" in Firebug (where .categoryListBoxContents shows but does nothing).
On another topic, I notice that you have a "Powered by American WebMakers" link at the bottom of your page. It is ok to say "Site designed by..." or the like, but "Powered by" is a claim that you wrote the Zen Cart software and is not kosher.