Board Thread:Support Requests - Designing Your Wikia/@comment-26537866-20150807170801/@comment-5590118-20150807171226

The categories on a page will either be at the bottom of the page if you are using the source editor or, if you are using the classic editor, they will be in the category module to the left of the editing box.

Unfortunately I cannot help you in regards to the visual editor as I do not use it.