For the life of me I could not figure out how to let user add wiki page to a category, or change cateogry, the category tab was just not showing up at all during edit or creation. I found the answer here by the solution above by andig, he/she stated:
Admin -> Permissions -> tiki-section -> set the right Can change the categories of the object (tiki_p_modify_object_categories) for the relevant user-group.
This worked beautifully in Tiki14.0.
Thanks for maintaining these archives of information, the old info still applies today!