> Is this because my server is a "development/test" server and does not have an SSL yet?
It might be because of absence of SSL on your X-Cart server or because X-Payments can't get through firewall to your X-cart installation or because you have X-Cart installed "locally".
Where do you have your X-Payments installed? If you do not want to post it publicly PM me!
Also
http://help.x-cart.com/index.php?title=X-Payments:Troubleshooting#.22X-Payments_tried_to_notify_the_store_...2_notification