At that line of that file, there shouldn't be a selCat value, only a selCatOpt. Is the error referring to that variable? What category are you choosing in the Menu Writer Category select list? Try selecting a different option from the select list.
When you say rebuild a css menu, does that mean that you are editing an existing menu or are you trying to recreate a menu you've already created using Menu Writer?
Are you running this on a page that is contained within the current site in Dreamweaver?