Unable to authorize my ereader (Kobo).

Cannot authorize my Kobo Glo HD with Adobe.  I am at wit's end.  I thought that I had succeeded a little while ago but still cannot read book.  I have downloaded ADE.  I can read the books on my iMac but impossible to transfer to reader as it is not authorized.   Just to let you know, I am not very good with computers and, if you have a solution for me, I will need a very detailed explanation.