I would also check with the library from which you are trying to download the ebooks from. The library subscribes to an ebook service and may have to authorize your library account or they will be more familiar with the service and issues that can arise.
Is it a Nook eReader or a tablet?
If a tablet, what software are you using on it.
If an eReader, are yuou goint via ADE (Digital Editions)
In any case, the userid and password it will want are for your Adobe account;
the same you use to get onto the Adobe web site.