If you'd like to integrate Mailchimp, Aweber, GetResponse, just follow these steps:
License your site
Add a form to your site
Click on 'Send'
Choose 'add user to a mailing list' from the drop down menu >
Click on 'Configure'
Click on 'Settings'
Click on 'Setup'
Choose Mailchimp from the drop down list
Enter your username and API key
Don't know your API key?
See here on how to find it - http://kb.mailchimp.com/integrations/api-integrations/about-api-keys