Go to digital editions, and use menu/Help/Authorize Computer (Library/Authorize Computer on the older more reliable v1.7.2)
Then with the device plugged in and showing in left panel of ADE use menu/Help/Authorize Device (Library/Authorize Device on v1.7.2)
Sometimes ADE gets confused registration. In that case you may need to deregister ADE (ctrl-shift-D, cmd-shift-D on Mac)
and device (ctrl-shift-E, cmd-shift-E on Mac) before you can do the registration correctly.