Uplay can be accessed from the main menu. Once you have registered for Uplay, you can spend Uplay points to purchase the following bonuses.

Assassin's Creed 3 Theme: Purchase for 10 Uplay points.
Multiplayer Profile Items and Pouch Upgrade: Purchase for 20 Uplay points.
Ezio's Oufit: Purchase for 30 Uplay points.
Multiplayer Profile Items and Night Stalker Outfit: Purchase for 40 Uplay points.