Hi
You first need to get your shop categories from eBay into Auctiva. Go to "My Account" and "Settings". Under "Preferences", click "eBay Settings". This will show you a screen which should have a link which says "Update Store Categories From eBay". Click that and the next time you go into a listing it should show the options to choose shop categories, just under the Category IDs.
Cheers
Geoff