I have an older Kobo, got it in 2010. I cannot open epub public library books with ADE

Need advice on how to open epub documents on my Kobo.  My Kobo ereader (it has wifi) will only open books that I have "bought" through Kobo.  I have downloaded ADE and have authorized it.  I can save the library books as epub documents, but they will not open with ADE when I click on them.  I get a message saying that ADE does not recognize this type of file or it may be damaged.  Any suggestions would be appreciated.