Check this url (www.linuxstall.com). The top navigation bar has one page (about us). I want the categories there, not pages. How can I do that?
If you are using the nav menus you can just add specific categories from your administration panel in Apperence->menus. If you don't see the categories box open "screen options" and check them to display. If you want dinamicly add them you will need to write a plugin.