IMPORTANT: MailChimp for WordPress plugin must be installed and activated in order to display the newsletter bar on your site.
Here's the HTML code we use on our demo site:
<input type="email" name="EMAIL" placeholder="Your e-mail address" required /><input type="submit" value="SUBSCRIBE" />
You should place the MailChimp shortcode into the "Shortcode" field on your customizer. Please go to "Appearance > Customize > 12. Footer > 12.5. Newsletter Bar" and you'll see it over there.
If you have troubles regarding the usage of MailChimp, please visit this page to find out everything about it.