No problem! To list an item within eBay Motors, open the "New Listing" page and choose "eBay Motors" from the "eBay site" pull-down menu on the upper-right of the Auctiva lister page. This will force the lister page specific to eBay Motors to load from which you will be able to select categories from within that eBay site.
If you have any additional questions or concerns, please let us know. Good luck!