Hi there kriesi,
I'm facing 2 problems here.
#1 the lightbox didn't work well as you can see here - http://7theme.com/2010/miscellaneous/7theme-com-is-newly-born/
#2 blog tab didn't show anything.
As you can see http://7theme.com/ here, the blog tab shows "Nothing Found" IF you click on it.
I've already did as what your tutorial write on. Creating a page name 'blog' & set it as Blog Page via Blog Options.
I just think it should be - make a 'category' named blog NOT 'page'.
Please help me how to do it your way.