If you haven't got an Adobe ID, get one (free) from the Adobe website. Try to login, and one option will be 'no userid, get a new one' or similar.
Remove any previous registration first: ctrl-shift-D to ADE (Digital Editions), and restart ADE.
Now register ADE with your Adobe ID. menu/Library/Authorize Computer.
You then need to pass that ID onto the kobo; while it is plugged in and ADE is running, menu/library/Authorize device.
Then it should all work.
May not work with the book you have already tried it with.
If not, borrow another book and try with that.
If the first one doesn't work and the second does, you won't be able to return the first one, you'll just have to wait for the loan to expire.