[topic moved to ADE forum]
I'm giving the same advice a few times on different threads, so I hope it's right.
Digital Editions recognizes most dedicated eBook readers (as in the list) directly so they show up in ADE,
but does not recognize tablets that have eBook capabilities.
This includes android tablets (probably including KoboArc), and ios tablets such as iPad.
If you have a tablet, you need to install an eBook reading app on it: I suggest Bluefire.
Make sure Bluefire app is registered to the same AdobeID as your ADE.
You can then copy epub files from your PC/Mac using a file manager and read them using Bluefire.
(Apple makes it more difficult, you need iTunes, Dropbox or similar to copy).
If the books are not DRM, there is no need to use ADE at all;
either download them direct to the tablet and read on Bluefire, or download to the PC/Mac and copy.
For DRM library books you will find an Overdrive app that allows books to be checked out, downloaded and read directly on an android/ios device.
There are also dedicated apps from various vendors.