Sorry, how should a backer specify which add-ons to include???
At this point, simply include the total of your Reward level, Shipping fee, and Add-ons in your pledge.
For example, if you wanted the Leader ($35) level reward, lived in the US ($5), and wanted to add-on a Robot ($10) - your pledge total would be $50.
You will receive a "survey" a little after the Kickstarter officially ends. This survey is used to confirm your shipping address, email, and exactly which add-ons you want to include.
I hope that helps!