Your post is a bit unclear: do you mean you are trying to download to the Kobo app on the iPad?
Some apps on the iPad don't have their settings within the app, but have them in a slot for the app under the iPad settings.
(Not sure why, seems crazy to me, but that's the way it is).
It may be that the Kobo is like that.
You could try installing the Bluefire app on the iPad. That has a place to use your AdobeID.