For most ereaders, plug it in by USB to a computer running ADE, then 'menu/Help/Authorize Device'. (Library/Authorize Device on 1.7.2)
If it is a tablet with ereader cababilities you may need to register it using the ereader app on the tablet.
Sometimes ADE 2 fails to recognize some devices, It may be worth trying older 1.7.2,
There are lots of bugs in ADE2.0 (and 2.0.1). Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172
Some people have found ADE trying to upgrade automatically.
It appears (not 100% sure) that if you install ADE2.0 as a new install (not as an upgrade) that your 1.7.2 will continue to run.
Probably best to say no if 2.0.x installation asks if you want to migrate your library.