I was about to add that I didn't have this in my settings, either. Then it occurred to me that when I originally had problems seeing the approval nodelet at all, the problem was solved by manually selecting which nodelets to show (rather than accepting the default). Well, the same thing works on "non-frontpage nodelets". Select which nodelets to add, and voila!, the settings page now lets you arrange them manually.

This solves my immediate problem, though there really isn't a good trail of breadcrumbs for the next puzzled monk to follow. Nothing I could find in the documentation described this settings page behavior.

Update: NOT solved. The approval nodelet seems determined to stay stuck below the XP nodelet, right in harm's way.