Hi Sam,
I just checked and you are currently using the Dozuki [http://www.dozuki.com/Sales/Plans/free|Open Source Plan]. The Open Source Plan does not include [[Help:Categorization|Nested Categories]]. The Open Source Plan was created to help companies and individuals create and display documentation for their open source projects and products.
If you would like to discuss setting up a trial of our Enterprise Plan (includes nested categorization) I would be happy to help you.