For Xara3D7, try Help > Update, then Close the "Welcome to the Xara Community" pop-up.
The Welcome! > Activate with serial number pop-up should then appear.

Xara3D6 & Xara3D5 can be registered by clicking Help > Register.

For earlier versions or if you cannot remember your email and password, contact the Xara sales team using the form at www.xara.com/contact for further instructions.

Acorn