To include add-ons, all you have to do is up the dollar value of your pledge, but keep the reward level the same. Once the Kickstarter has concluded, you'll get a chance to note which add-ons you want included for your money.
For example, if you're a Digital Supporter ($10 pledge level) and decide that you want to pay $5 extra for the Destination add-on, just increase your pledge to $15, but keep the selected level at Digital Supporter/$10.
Send me a message if you'd like some more clarification. Hurry up! Add-ons can only be added during the duration of the Kickstarter.