The iPad is an independent device rather than an eReader, so does not connect to ADE directly.
You need an DRM eReader app on the iPad such as Bluefire ~ you presumably have such an app or you couldn't have authorized the iPad?
To read files from PC/Mac on the iPad you need to sideload them, that is to transfer them outside the Adobe infrastructure.
For most devices this is simply a matter of copying the files with a file manager such as Windows Explorer or Finder.
Unfortunately, Apple has chosen to make it awkward to transfer files onth the iPad.
This must be done by some extra application run on the PC/Mac.
Three standard ones are iTunes, Dropbox and emailing the .epub file to yourself.
I'm using the Overdrive app, and their help pages indicate that I can then use Digital Editions to transfer. This is what I'm looking at: http://help.overdrive.com/article/0005/How-to-transfer-eBooks-to-an-eBook-reader-using-Ado be-Digital-Editions
I connect the ipad, then open ADE as instructed, but nothing appears under "devices:
I notice the line that says Note: If you don't see your device, you may need to install some additional software. Please check your eBook reader's documentation for more information. However, I can't find any reference to "additional software" in Overdrive or ADE help files or web pages.
I'm not itunes savvy, and I don't know how to transfer book files using itunes.
I think that page is an Overdrive page about transferring books to dedicated eReader devices.
It applies to all Adobe DRM books, including Overdrive library books, but does not apply to the Overdrive app running on a tablet such as iPad.
The additional software bit is because some dedicated eReaders (Sony PRS-350 for example) require special drivers to be seen by ADE.
I'm not sure if ADE can transfer to any eReader app on any tablet (if anyone can fonfirm yes or no I'd be interested).
On sensible tablets, the alternative is to use a file manager to copy the files.
Unfortunately, as I said, Apple likes to make life hard for iPad users to perform any simple data transfer.
Transfer of photos is similarly convoluted.
This is a case where Adobe/ADE is not at all to blame: the fault is 100% with Apple's absurdly protected environment.