Based on the excerpt taken from this page http://www.wordpress.org/plugins/mailchimp-wp below, the post boxes that can be added with a shortcode to automatically at the bottom of the post/page, does is broken.
MailChimp Forms by Optin Cat – Features
Here are some other cool features that MailChimp Forms by Optin Cat comes with
Responsive design. Your MailChimp forms will never look out of place, no matter which device your visitors are using.
Post boxes can be added with a shortcode or automatically at the bottom of posts/pages.
Lots of customization options. Choose between font sizes, colors and more.
Intuitive and elegant interface that keeps things quick and easy.
Analytics feature let’s you know which MailChimp forms work and which ones don’t.
Conversion optimization best practices built-in – MailChimp Forms by Optin Cat is the work of experienced marketers who know what makes people convert.
