I just bought a copy of ACII at the store today and installed it on my computer without being asked for the product key.
When I selected tried to log in on the launcher with my Uplay account, it said something like "Login Failed. You have to log in online once before you can play offline." (roughly translating from german).
When I selected "Register" in the CD menu, I only was transferred to a website for a few seconds before it took me back to the Ubisoft Store page.
So the question is:
Where do I put in the Product Key of my game?