If you have purchased through Steam, you can find your key by right-clicking Assassin's Creed Revelations and selecting View game CD key.
Once you have your CD key, please continue with the steps below.
1. Load up Assassin's Creed Revelations and access the Extras menu.
2. Select Exclusive Content.
3. Enter your key on the following menu and press Accept.
4. You will have 4 new characters added to your multiplayer menu (the Privateer, the Corsair, the Brigand, and the Gladiator).