So the solution was to go to "App settings", type in my Adobe ID and password and "Authorize" the App. However, I was asked to authorize my App every few days or so, an annoyance. Why did I have to authorize every few days, why didn't Overdrive remember that I had already authorized the App?
After a few weeks, the annoyance became a problem....When I attempted to authorize the app, I received a error message saying that there was "Too many Authorizations". Blocked-the book would not open, nothing I could do...
Of course the problem happened when I was on vacation and finishing up reading the eBook Cutting for Stone, and I was almost at the end of the book, very frustrating.
I did some quick browsing and found that many people reported having the same problem, and the solution was to call Adobe and have them re-set the authorization count.
I was able to open a chat window with Adobe Support, and got a good support person who reset the count for me, and that allowed me to authorize the app, and finish reading my book.
Since that time, I have not had to authorize the app again...perhaps someone wised up and fixed the underlying problem.