If it is Adobe Digital Editions (ADE) software you want to register, log onto Adobe.com and sign up for an Adobe ID.
That is not initially associated with any device/serial number.
Then associate (register/authorize) ADE and your computer with the ID, menu/Help/Authorize Computer.
Then plug in the book reader and it should show up in the left panel of ADE.
Then associate (register/authorize) your device with the ID, menu/Help/Authorize Device.
(On the superior older v1.7.2 it is menu/Library/...)