[kumaryu]

When starting to use a new Kobo ebook reader or after a hardware reset, you need to "register" a user for the device using the Kobo Desktop application on your PC or Macintosh. *

Registration using the Kobo Desktop application "activates" the database on the device to recognise the files that you copy or transfer onto the device. Without this activation, the files that you transfer to the device using Calibre or file copy will not appear in the "library" of your device.

*There are unoffical ways to "hack" this activation but unless you have specific reasons for doing so, I would suggest using the Kobo Desktop application first.