So, we're using the new beta "Add-Ons" feature from Kickstarter. Honestly, it's pretty great.
However, some folks have reported having trouble seeing that option on their PC or Phone. All I can suggest is trying on a different platform; try a different browser, or if you did your pledge on the PC, try EDITING it on your phone. (Or vise-versa.)
It's in there... many folks have succeeded... but clearly it's giving some folks some troubles.