To set up eBay, click on Channels > eBay in your admin. From there,
Enter your Account Name
click on the "Setup eBay" link that appears below.
The following should come up:
Make sure to finish step 1 completely before moving on to step 2.
*Note: Once this has been completed, please see the following articles:
A full step-by-step setup is outlined in the following video: