MailChimp Integration with Membership

Login to your mailchimp account at http://mailchimp.com
From the left main menu, click on your Username, then select “Account” in the flyout menu.
From the account page select “Extras” -> “API Keys”.
Your API Key will be listed in the table labeled “Your API Keys”.
Copy / Paste your API key into the “MailChimp API key” field in MailChimp section of Membership Subscriptions Settings.