Double-click on a volume to open the categories window and the files window.

Renaming a category
To rename an existing category, click on it once to select it and a second time to make its name editable.  Change its name and then press the <return> or <enter> key to leave the edit mode.
When you are in edit mode, you can edit the next category by typing the <tab> key.

Creating a new category
Use the New Category menu to create a new category.  The edit mode is automatically invoked so you can change its name directly.

Modifying categories settings