Kickstarter does not make this obvious how to do it, but here is a step by step.

1. Go to the campaign page: https://www.kickstarter.com/projects/nerdgear/commander-essentials-kit-for-magic-the-gathering-edh

2. Click on "Manage your pledge" button at the top

3. Click on the "Change your pledge" button

4. Your existing pledge should be highlighted with blue, in the box Click on the "Pledge $xx" button

5. Now you should see the addons if you scroll down a bit.