Registrations and Domains
We would like to clear up the difference between a Registration and a Domain, for every website there are pretty much two parts. The Registration and the Domain, The Registrations or Domain Registrations as some call it can be considered a "Name Tag" for the website.
and consider the Domain like a box where you place all of your website files and content in.
When a visitor looks up your website name, the internet looks for the Registration (name) then the Registration points the visitor's browser to your Domain that is hosted here with us.
This would explain why if you add a Domain but do not register a Registration for that domain, it will not work because you will have the box but nothing pointing to it and if you register a Registration but do not setup the Domain for that registration, it will also not work because you have the name but it is not pointing to any box/files.