Miva 10 is here! See what's new | View Full Feature List
There's a number of things that you need to do before you can create an eBay listing from the admin interface. See An Overview of Marketplaces - eBay.
One important task is to link your Miva Merchant store to your eBay store, and you do that in the Accounts tab. See To Link Your Miva Merchant Store to Your eBay Seller Account.