In the "My Library / Purchased" section of the Kobo Books website
http://www.kobobooks.com/library/purchased.html you will see all books you have bought. Besides each book you will see a blue box that will read
ADOBE DRM EPUB or
ADOBE DRM PDF (not sure of the exact wording for the PDF).
Clickinng on this will download an ACSM file. This in turn when opened on your PC by ADE will download either the ePub or PDF format of the book.
With ADE open and your Kobo attached to the PC you should be able to "drag" the book to the Kobo (again, within ADE).