If you wish to add users to a mailchimp mailing list using a form on your site you can do so by adding a form to any page on your site, or using our "Pop Up" feature.
To integrate Mailchimp
- Add a Form to any page of your site or add a Pop Up form
- Click on Send to "edit"
- Choose 'add user to a mailing list' from the drop down menu
- Click on 'Configure'
- On 'settings' click on 'Setup'
- Choose Mailchimp from the drop down list and add your username and API key.
See their article below to find it - http://kb.mailchimp.com/integrations/api-integrations/about-api-keys