Now that the MailChimp Add-On is active you will need to setup your MailChimp account to integrate with MailChimp. The MailChimp Add-On adds a new link to the "Settings" menu under "Forms". Go to the "Settings" page and click on the "MailChimp" link.
On the MailChimp Settings page you will need to enter a valid MailChimp API key. If you do not have a MailChimp account you may sign up for one here. To generate an API key follow these instructions. If you have entered a valid API key you may then proceed to setup your MailChimp feeds.
Setup MailChimp Feeds
MailChimp: Add Feed
Your MailChimp Feeds allow you to integrate a form with your MailChimp account to feed user information in real time when a form is submitted. This allows you to automatically add subscribers to your mailing list when a form is submitted.
Now that you have setup a MailChimp feed, when the form you selected is submitted it will automatically send the data you have selected to the MailChimp List you have setup to receive data.
Please keep in mind that if you have selected "Double Opt-In" that a subscriber will not be added to your list until they click on the opt-in link in the automated email they receive from MailChimp.