Problems adding menu items

Hi everybody,
I’m a beginner with WP. I was trying to add some menu items to my new menus (I am working with the ‘twenty ten’ theme).
Under Appearance > Menu, I tried to add some menu elements, e.g. Categories, on the left, to the menu defined on the right of the panel.
Neither drag-and-drop nor pressing ‘Add to menu’ work. Instead, I am being redirected to an empty page: //wp-admin/nav-menus.php

I tried a fix proposed by someone on this forum, namely to replace the entire js source code in ../wp-admin/js/nav-menu.js.
Didn’t help.

Thanks for any help.
Greg