header link background

please look at:

katerone.com/word

i managed to intergrate punBB into wordpress with the k2 theme, it is working quite well, (still in the process of modifying it), but there is one thing i can't figure out.

when you click on "blog" the background around the word "blog" goes white, to show that it is selected, but when you click on "forum" i want the same effect (white backtground around word "forum") but this doesn't happen, can anyone tell me what to look for or where i can fix this.

I'm fairly new at this.