If you're using the old blue & white "default style" there is no site navigation available.

Personally I prefer this look than the new "fromBasics Dark" style as the latter seems too compact and cluttered for my liking, although it does include the site nav. Hopefully Reggie will add the site nav to the default blue/white style, or perhaps tweakl the "fromBasics Dark" style to improve usability and make it easier on the eye. Or both

You can change your style using the list box located at the bottom left of the forum list page.