I downloaded the .NET ASP Contact Forms, migrated the project to VS 2012, added my APIKey, username and password to the web.config. When I compile I get a 401 Unauthorized. My account is a 60 day demo.
401 Unauthorized may indicate that you are using a different URI than you should be. If your username, password, and APIKey are all correct, can you make sure your username is also referenced in the URI you are trying to get/post/put/delete to?
Also, you may need to list your username as APIKey%username when you try to access your account. Let me know if that helps.
Thank you for contacting Constant Contact, and have a wonderful day!