If you look in this file: app/design/frontend/default/default/template/catalog/navigation/top.phtml, look for the following code:
<?php foreach ($this->getStoreCategories() as $_category): ?>
<?php echo $this->drawItem($_category) ?>
<?php endforeach ?>
After this you could put <li><a href="yourlinkhere">you title here</a></li>
The only problem I’m having with this method is that the active state does not appear when you follow the link to the specific page.
The other option is to go into the admin under Catalogue > Manage Catagories. On the left-hand side click the “Add New” button. There should be many posts in the forums if you do a proper search