In Wixi, every ''folder'' in the tree is a category. Adding pages to a category can be done by right-clicking on a category. Select the append to page option. This will append a rule to the page wich holds a category mark. You may add more then one category.
Note that if you are adding more than one category you may do this in one line by using a comma after the first category. For example:
%!wixi-category: About, Info, Etc
example