When it comes to the DataAssist wizard for this setup you should create the pages you will need to manage this table. The pages that DataAssist creates are a typical set of these pages, unless you have a reason for not creating any of these pages you should make them all.
When it comes to configuring the individual steps in the wizard you should use the help button, it will take you to the help page specific to that step of the wizard. In here you are just creating the settings for your insert, update, search, results, and delete pages so long as you can insert new subcats, update them and search on them if needed you will be able to make use of them.
When it comes to associating the category with this sub cat you will need to do this on the insert and update pages first after they are created from the wizard. You will need to replace the field you have for the category and add in a recordset to select all of the available categories. You will replace the category input with a select list, and set the select list to be dynamic from the category recordset. This part will make a lot more sense when you have the completed pages to work on.
If you have any general questions about using DataAssist take a look around the forums here, many users have been through the same things.