Admin Forum:Adding Sub-Forums

I have used the guide on how to set up Sub Forums on my wiki  >>>(http://hogwartsworldofwitchcraftwizardry.wikia.com/wiki/Help:Wiki-style_forums)                                                      but I do not understand step 3. Please could someone explain it to me and also do I really need to do step 3? When I try to add a topic it does not show up, is that becuase I need to do step 3?