Adobe Digital Editions 4.5

I have had to purchase a new computer. On my old desk top I was using Digital Editions 3.0 to open books from my public library. This permitted me to download them onto my KOBO reader. Now, as 3.0 is not used any more, I have had to download 4.5. When I download books from my library, they will not go into the 4.5 edition because it is not authorized. Yet, when I try to authorize it it comes back and tells me the computer is already authorized. I am getting nowhere. I have contacted Adobe and they have told me to put the question out here and somebody would contact me. Can anybody please help me out here.