Kobo Support told me to: ...
10. Connect your eReader to your computer and tap Connect.
11. In ADE, click the gear icon beside Devices. Click Authorize Device.
12. Select the account that you'd like to authorize your eReader with. Click Authorize Device.
13. Click OK to complete the process.
BUT at step 11 there is no "gear icon" or "Devices" in my ADE 3.0. I only see File, Library, Help on the Menu Bar, and "Bookshelves" below that. There seems to be no option to authorize any eReader device.