Can't get adobe digital editions to work

I'm trying to use Digital Editions 4.5 on both my PC and my iPad. I've run into the complication that at various times I've used different e-mail addresses and passwords so I made many efforts to completely de-authorize  Adobe Digital Edition and start over with one email address and one password. But I can't get it to work properly. All I want is to be able to borrow books from a library and it doesn't seem as if it should be so hard. Can you please help me? Thanks