Tab Order in Mantra

I’m new to WordPress, so bear with me if this is a dumb question. I am creating pages, and they are happily showing up as tabs in the header. But how do I control the order of them? After Home, they apparently are displayed in the order I create them (newest on the left, next to Home). Can I adjust the order? In Appearance->Menus, I can see my pages in the Pages section, but the whole thing is grayed out – the only thing functional there is the “Create Menu” button. I get the impression from Google searches that this question is related more to the theme than core WP functionality, so I’m posting here.

The site can be found at http://japan.deafmin.org/wordpress/, but it’s in Japanese, so that may not be helpful to you.