Click on Channels -> Add New Channel menu at the left menu bar.
Select the type eBay from the left tabs as shown below.
Choose the eBay site belongs to your eBay account and click on the button Click Here To Connect.
You will be redirected to eBay website for your authorization to connect your eBay account. Enter your eBay login details and click on Sign in button.
Now click on Agree button as shown below to accept the permissions.
You will be redirected back to the SKUBubble once this process of connecting eBay seller account gets completed.