Create an account or log in with the existing account from Uplay;
Select the "GAMES" item at the top menu;
Click the "Activate product" option and apply the key code received after the payment in the popuped window;
After the activation the game will appear in the list of the games and you will be able to download it. The process of installing (downloading) takes approximately 10-60 minutes (depending on the existing internet-channel).