Hi Melissa!
First, go to the "My Account" tab in Auctiva and then "Setting"->"Ebay Settings". Here, clink on the lick to "Update your Store Categories from Ebay".
Now you can go to your listings and use the scroll down menus (under where you chose an ebay category for your item) to choose ebay store categories.
Hope that helps...if not, let us know!
Marcia