To send the submitted date to your MailChimp account you should have the following information:
Your API key from here - http://admin.mailchimp.com/account/api
List unique id from here - http://admin.mailchimp.com/lists/
First you should open the settings of the project and then open the "Forms" tab, then choose MailChimp and input the API key in the corresponding field at the end of the page.
After that you should open your project inside the builder and click on the "Form Settings" button in the settings of the column which contains the form, then from the right sidebar open the "Settings" tab and choose "MailChimp" as form type and input the list id in the "List ID" field. And you should make sure that each field of the form has the same name in your MailChimp account, to change field's name simply hover on the field inside the builder and click on the settings button on it, the from the popup you can change field's name, placeholder and set if it's required or not.