It depends on whether the Kobo ereader has wireless capabilities. if it
does, then you can go to the Kobo bookstore via wireless, and activate it
there. Be sure to use your Adobe ID and password to activate it, because
Adobe Digital Editions (ADE) will be looking for that information when you
start it up.
If your Kobo ereader isn't wireless, then you can attach it to your computer
via the USB port, and go to the Kobo bookstore, where you can activate it in
a similar way.
Hope this helps!