I cannot access the desktop app using my ebay username as I was directed in the email I received. But when I use my email address and password I can access the app. But when I use the registration key (help > enter key) and log out and then log in again, I still get the message that I am using the evaluation version which I should not be getting after I enter the registration key. I suspect that if I could access the app with my ebay user id and then use the registration key I would not be getting that message. Can you help me out? Many thanks.

I assume you mean you can access it, but you can't login to your sniper's account.
If you can't login using "brsm-7484" as your username, please reset your sniper's password at https://www.myibidder.com/login/reset