Documents can be added to the Horizontal Navigation by placing them in the Sitemap under "Main navigation":
Within the document itself, the options "Show in navigation" and "Show in main navigation" must be selection in the Navigation area:
The text which appears in the navigation is defined under "Navigation-Text" .
Documents which lie in the first level underneath "Main navigation" are shown in the Horizontal Navigation:
Documents which lie in the first or second levels underneath this appear Vertical Navigation: