ADE is very good at getting its activation screwed up.
Try explicitly to deactivate ADE. In Library view, hit ctrl-shift-D (cmd-shift-D if on Mac). Then restart ADE and try to reactivate.
It is likely that the Kobo readers are still properly activated and if the above works to get ADE activated again things will start working.
If not, you also need to deactivate and reactivate the Kobos in a similar way.
Plug a Kobos in on USB. Run ADE. In Library view, hit ctrl-shift-E (cmd-shift-E if on Mac). Restart ADE, and choose menu/Library/Activate Device (or Authorize Device).
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There are lots of bugs in ADE2.0. If the above doesn't sort things out, you may find that it is best to reinstall v1.7.2.
(You may need to go through the deactivate/reactivate palaver again after that.)
It is possible to have ADE 2.0 and ADE 1.7.2 installed and registered on the same computer if you like: they don't share libraries though.
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
http://helpx.adobe.com/digital-editions/kb/cant-install-digital-editio ns.html
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