I want to add a third level category to my products, but ZC only allows two levels, as far as I can tell. How can I add a third level?

To see what I'm talking about, click here:
http://www.abigailmiller.com/store/i...if3l898tte0jg4

That takes you to my "Sheet Music" page. Then I have two sub-categories: "Sheet Music Books" and "Sheet Music Electronic Downloads." I want to add categories WITHIN the "Sheet Music Electronic Downloads" section. All SC seems to let me do is add a new product at that level. No button for a new category.

Any help is appreciated. Thanks!