To configure eBay with Odoo, you will first need the eBay module installed. Then go to e-commerce->eBay->Configuration Settings->eBay Instances. Click on create and add your Dev ID, Cert ID, and Token values. If you do not have these please contact support.
After saving your new instance, check the box next to it and select "Create eBay Shop" from the More menu. Fill out the appropriate information for your business on this form and click "Create eBay Shop"