How do I solve my problems downloading and reading EPUB files on computer and NOOK

I recently replaced two computers and now I can't download books from my public library. I had originally downloaded ADE 4.5 and I was able to download the EPUB files but when I transferred the files to my Nook I couldn't read them. The folks at NOOK said it was because the NOOK doesn't support ADE 4.5 so I uninstalled 4.5 and installed ADE 3.0.  Now I can't download the files to my computer.  When try I get  Check Activation error code.