After signing into the Ebay Home Page, click on My Ebay on the upper right of the screen, this will bring up a list including "Summary". Click on that which will bring up the next page (of course). At the upper left just above the tabs titled "Activity", "Messages", etc. you'll see your user name. Click on that and your Feedback Page will come up.
It's pretty clear that yur desired user name i.e. Ebaby is already in use... change it to something else and try again... (If your e-mail address is already registered that's a clue that you've registered previously... perhaps you forgot about it.)