You must make sure your copy of ADE is registered with an explicit Adobe ID, and that the Kobo is also registered with the same ID.
If you haven't done so already,
- Visit adobe.com and try to log in; that will give you the chance to register a (free) Adobe ID.
- Then menu/Help/Authorize Computer and enter the ID (menu/Library/Authorise Computer on the less buggy older version of ADE, v1.7.2)
- Then with Kobo plugged in, menu/Help/Authorize Device (menu/Library/Authorise Device on v1.7.2)
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There are lots of bugs in ADE2.0.
One is that it refuses to work at all with some devices,
another that it refuses to work for library books with some devices (including many Sony eReaders).
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.
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