I installed a new WP site with 3.1.2. The way I set menu/header on my other sites was with the widget "Header: Nav Menu". When I go to that widget now, clicking the + sign to the left of "Items" doesn't add the new item.
I'm thinking it's a browser compatibility problem, but I've tried FireFox 4, Chrome 11, and IE 8.
Update 05/25 with Sample picture:
I believe what used to happen is that when I clicked the plus-sign (pointed to by red-arrow), a new gray box would appear, where I could enter a new URL. But absolutely nothing is happening when I click the plus-sign.