1. Go to the right hand corner and click "manage."
2. Click on "change your pledge."
3. Add the add-on cost to the current total and continue.
4. At the end of the campaign you will get a survey asking for your address & reward info. Include the add-ons.
Note that these add-ons are available with any reward in the USA. If you are outside of the USA, you must have a reward worth $14 or more to use an add-on, and you must add $25 shipping to add one or more jumbo version of the game.
You may not get more than ten of any type of reward, such as ten copies of Crazier Eights: Camelot.