where did you buy the game from ?
thats who you have to contact for a new copy \ refund
if you bought it used it may be worthless
cd keys are good for 1 use only
after that they are bound to the Uplay account they were activated on
if you bought the game from a CD key site contact them
also make sure you have down loaded and installed the latest version of the Uplay Client to activate your game on
you can get it here https://uplay.ubi.com/
the version that comes on the Disks will be OUTDATED
Far Cry 4 shipped on Nov 18th 2014 ..shipped version was 1.2.0
current versions is 1.10.0 ..so any data on your DVD set probably will not be used at all ..the game will download the updated version via Uplay, once you enter in a valid product key and create a uplay account