How do I add a post to a menu

I’m trying to get WordPress to highlight a specific item in my menu when a post (any post) is viewed. I’m thinking that adding the post to the menu item, then suppressing display of sub-menus might help, but my ‘Menus configuration’ page doesn’t show posts as items to add to the menu.

Does anyone know why that is, or if there’s a better alternative to this method?

I’m using a copy of the default TwentyTen theme which calls wp_nav_menu in header.php.

You should be able to take the body class (single) and the nav item class (to be determined) and specify the style you’d like to show in the stylesheet. Something like this:

.single .topnav-item-29 {color: #fff; background: #333;}