Can't download library books to my ADE, v. 3.0 from Mac OS X, with Yosemite download

Thank you. I did find a book that i had checked out from the library in the documents file. however, the book wants to open in iBooks. i want the book to go to my Simple Touch Nook and i thought that i could only do that through ADE, v. 3, which i have downloaded (several times). however i can’t get the library books to download or go to ADE anymore. I reauthorized the computer, using an Overdrive authorization, but i still can’t get books to download to ADE. i believe that before all this trouble, i had an ADE authorization. I was wondering if i should de-authorize the computer (but don’t know how to do that) and re-insert an ADE authorization. (but of course i don’t really know how to do that either). i have spent so much time one this matter that i can’t remember what i’ve done. Nook and Apple say that this is an ADE problem, and i have reviewed the discussions in the Forums, but so far i haven’t found anything exactly on point — or perhaps i just don’t recognize an answer that is staring me in the face.