If you are new to Kickstarter, the add-ons are the most confusing thing in terms of pledging since they do not work on a "cart" system like everything else does on the internet. In order to "purchase" an Add-on, simply add the cost of the add-on(s) you would like to your pledge amount.
Example 1: You live in the US and want to add-on the Juggernaut. You simply add $10 to your pledge amount as there is no charge for shipping for this add-on.
Example 2: You live in Taiwan and you chose the Founders Pledge and would like to add another copy of the Founders Edition and two Juggernaut add-ons. You would add an additional $59 for the game, $2 for shipping and $20 for the Juggernaut add-ons, increasing your pledge by $81.
At some point after the campaign, we will send you an email with a link to the pledge manager, in here you will "spend" that extra money you pledged letting us know what add-ons you wanted before we ship your stuff.