When you wish to add-on to you pledge, here is how you do it:
Simply increase your pledge (with the Manage My Pledge Button) the appropriate amount of money for the add-on you wish to purchase. Be sure to keep any shipping costs in mind as needed for International folks. After the Kickstarter closes, you will list your add-ons during the Post Kickstarter Survey for exactly what you added. All you have to do in the meantime is increase your pledge amount.