you should have a categoris and itemcategories table.
on the items insert page, create the checkbox section of the categories just like you did for the genres
and add another multiple insert behavior to insert the selected categories to the itemcatagories table.
on the update page, do the same, set up the checkboxes for categories the same as you did for the genres, and add the manage relational table behavior to manage the itemcategories table.