Kickstarter isn't a store, it only lets you select from our pre-built pledges, and won't let you pick and choose items. However, it will let you add any amount of money to your pledge. So if you're looking to add any of the Add-ons, or add additional copies to your pledge, just add the amount of money you need for those items. for example, if you've pledged for Fear Your Future, but also want a second copy of it, just add $29 to your pledge.
After the campaign closes, you'll be sent an email inviting you to BackerKit. BackerKit is a separate site that allows you to update you Kickstarter pledge and set your address.
When you get into Backerkit, you'll see that have an amount of money credited in your account. You will then be able to select the add-ons or extras sets as you'd like. This works very much like a web cart for a site like Amazon.
So don't worry about selecting the add-ons or extra sets at this moment, you'll be able to do that later in a much more user friendly interface. At this time all you need to do is add the money needed to purchase the add-ons or additional sets. We'll take care of the organization of the funds later.