You can do this using jQuery. You have monitor the click events for the pledge links then apply the value to the field in your form. You may have a problem with your site. When I was checking your page, you have script files that are not being loaded because of a mismatched mime type. You need to fix that because any js error could prevent jQuery from working.

Thanks @Victor Font and @Rob LeVine for taking time to comment. I did end up solving this by just using the shortcode [get param="amount"] as the default value in the donation amount field, and forcing the page to reload. I found this somewhere in the Formidable documentation but it took quite a bit of hunting. Here is the live site.