To authorise your computer with an Adobe ID, start by creating an Adobe account.
Then, in the Adobe Digital Editions menu, go to Help > Computer Authorisation.
In the “eBook Supplier” field, choose “Adobe ID”. Then enter the email and password for your Adobe account and click on “Authorise”.
Once the computer has been authorised, click on “OK”.
In both cases, you should not check the box “I want to authorise my computer without ID” because this would limit the reading of your ebooks to only your computer (you will then receive this error message: “CE_COPY_NOT_ALLOWED”).