You have to go to the My Library
section of the Kobo web site where you should see the book(s) you have purchased. Next to the book is a blue button "ADOBE DRM EPUB" that when clicked will download an ACSM file. This file, when opened via Adobe Digital Editions (ADE) will download the ePub which will contain DRM.
Google for Apprentice Alf for instructions on how to remove the DRM and then you can add it to calibre and do whatever you like with it.