Why ADE doesn't specify the information it requires to unlock a book is beyond me.
As you will see from other posts, I think ADE is a terrible piece of software.
However, in this particular case it is somewhat justified.
Registering with the Adobe ID is relatively straightforward (unless something goes wrong), and works for most books.
However, some suppliers (such as B&N it appears) choose to use their own userid/password combination.
ADE won't know how each supplier sets up userid/password.
It could give a more useful generic hint, such as
'This book requires authorization with a vendor ID and password. Check with the the vendor how this information should be provided.'
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ADE is in an awkward position, as it needs to cater for people with few computer skills
(such as kate682: I am a 70year old woman with few computer smarts.).
As such, it should be exemplory in its user interface and the way it handles unexpected situations.
Instead, it is an apalling load of rubbish that is awkward for professional users even when everything goes OK,
is riddled with bugs, and creates its own 'unexpected' situations where they have no need to occur.
As an (otherwise) reputable software company, Adobe should be ashamed of themselves.