If you used used the Kobo desktop application to obtain the book, it has been downloaded in Kobo's proprietary format (often called kePub) and can not be used by any other application.
Head back to their web site, and go to "My Library". There you should see the book you bought and the option to download the EPub version; when you click it it will download an ACSM file that in turn you have to open via ADE.
Then all that is left is to remove DRM and convert to Mobil!
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