Can't read a book on my Kobo Aura HD device

I've downloaded a book from my local library. I can see it in Adobe Digital Editions and I was able to drag it to my Kobo but when I try to read it, I get the message that this document is protected by Adobe Digital Rights Management and it no currently authorized for use with my Adobe ID. I am able to read it on my computer. From all the help I've found it says to go to ADE and select Authorize Computer but that option is greyed out. What do I do now?