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.
Don't see the answer to your question? Ask the project creator directly.