The device has not been activated. ADE should ask if you want to activate it when you start ADE, but sometimes you have to explicitly choose 'Activate Device' or something similar in the Library dropdown menu.
The device has been activated, but somehow the job got half done. Deactivate the device completely (ctrl-shift-E to ADE with the device plugged in), restart ADE, and reactivate the device as in (1).
A more extreme version of (2). Only try this one if nothing else works. Deactivate the device completely (ctrl-shift-E to ADE with the device plugged in). Stop ADE. Look at the device listed in a file manager (eg Windows Explorer) as a external drive or whatever. Make sure you are seeing hidden files and folders. Delete the folder '.adobe-digital-editions' from the device.
ADE has been activated. but somehow the job got half done. Deactivate ADE completely (ctrl-shift-D to ADE), restart ADE, and activate ADE. (Library menu dropdown 'Authorize Computer' or similar if it doesn't ask automatically on restart of ADE.
(Probably worth repeating step 1 if you do this to make sure ADE and the device tie up properly)