Is the Kobo a dedicated ereader or a tablet?
For a dedicated ereader, it should show up in left hand panel of ADE under devices when plugged in to USB and turned on.
If so, use menu/Help/Authorize Device to authorize it.
If not, check in your file manager whether it shows up as a mounted drive there, and post back here.
For a tablet, run an ereader app and that will have authorization withing the app, probably in Settings somewhere.