masque, you need to persist with Magix. A number of customers fail to be registered properly in their database through no fault of their own.
In Xara handing over support to Magix, the whole database was reset so you may need to also push on that front too.

To get at your Serial number, you would normally press Shift+Ctrl+Alt+1 in your application and write that down. (Xara Pro+ is different as it synchs directly with Xara Cloud).
Failing that you need to locate your purchase email or search your emails for Xara Shop or Xara Online Shop.
I always try to keep two copies on two machines for this very purpose.

Quite sad really.

Acorn