GrandNode requires to enable plugin first. If plugin isn't enabled, tab for mapping won't be visible
There is an extra tab on Category Details. Use it to map store categories with eBay ones.
To set up appropriate category just choose it from tree list. It will be saved automatically.
Note: If you don't map category, you won't be able to export products from that category.
After successful mapping, you should see following window on product edit page: