Hi Kevin. We don't currently have a category search. There is a complete alphabetical list of categories here, but it's really long, so it's hard to look through. The best way to find a category is to click through from a higher level category. If you know the exact name, you can find it by typing it into the URL in this format: http://www.wikitree.com/wiki/Category:Cottingham, East Yorkshire
The key to being able to find categories through the hierarchy is that you have to categorise the categories. :) I edited yours to do so. I just added the line [[Category:East Riding of Yorkshire]] to the page. Now it shows up on the East Riding of Yorkshire category page.