in the main menu of assassin's creed 2, press Y to enter UPlay. then, go to actions and rewards (actions are basically achievements you have to complete for points, and rewards are the extra content you can get with UPlay points) and use the UPlay points there.
Uplay doesn't work right for me. From the page that lists the compatible games where you click the link for actions and rewards, when I click on that it just refreshes the game page. I can't get the action and rewards to open up for me.