Make sure both ADE (digital editions) and the Kobo are registered with the same Adobe ID.
If ADE is registered, then you need 'Libraray/Authorize Device' (on ADE1.7.2) or 'Help/Authorize Device) (on ADE2.0) to register the Kobo.
If that doesn't work ...
Sometimes ADE gets its registration/activation confused and in a semi-authorized state.
Unfortunately, it often then gives misleading error messages about what is wrong.
Try completely removing any authorization using ctrl-shift-D to ADE (cmd-shift-D if on Mac).
Restart ADE, and then reauthorize ADE with your Adobe ID.
After that, register the Kobo as above.