I haven't used ADE much but I've just found out there's lots of information available from Kobo about ADE here:
http://www.kobobooks.com/ade (including the download link for ADE)
Once you have installed ADE and authorized your Kobo, you can download the book from your browser (log in to your Kobo library, there's a link on the book's page). It will be opened in ADE and you can transfer it to your Kobo from there.
There's detailed information about the process on Kobo's ade page and I couldn't explain it better.