Ok, I figured out how Nitrado does the activation of the domain part and I was able to make my domain active.
I am assuming the following: (1) you purchased a domain through Nitrado- (2) you purchased webspace through Nitrado.
I was able to link the domain to the webspace in 30seconds with no issues. I was rather impressed how easy it was.
In you services page, see where the domain is. Click on the down arrow and select "allocate web package". This will take you to a page with a dropdown. Select the package you just had created. Select save or change. It will link the 2. Mine worked fine. Did you do that part? if so and you are still having an issue, you will have to wait for a Nitrado Support person respond to your ticket.
As for the DB+User. Their interface is not what I would expect. I know what you need but I do not see the options in the screen. I see where you create the DB and grant access to the outside. Another option should be there to create user of the DB so you can link the 2.
I see only this....
Password: the password you've chosen
You and I have figured out the DB creation. But I see nowhere to create the UN+Pass. In cPanel its right at the beginning.
Sadly for this part you will need Nitrado Support to respond to your ticket.