For STEAM users you will get the DLC auto, For uplay aswell, If you using retail version then you have to find the DLC on google, while ingame you search for icon that called V and fast travel there and then you will have access for the DLC.
i do not own the game or use uplay but from all i have read if your game is legit you should be able to get it as a free patch to your game so again if your game is legit and you are haveing trouble then maybe you should ask over at ubisoft for some answers
Its auto download and install for uplay and steam, just go to map of Paris, look for fast travel with the letter F on it. Also make sure you finish Sequence 4 because it wont let you go to dead king dlc map before that. Hope it helps