Originally posted by Mr_Shade:
If your 100% sure the code is correct [not confusing 1's with I's for example] - you have a few options.
You can either contact the retailer, who should offer to swap the game for you - so you have a new CD Key
You can contact Ubisoft Support - who will, after they verify the Key has not been used, supply you with a new one
You may need to provide photographic proof of purchase, so please be aware.
I personally would try the store first, if your able, since it may be quicker.