MailChimp API Key

To use MailChimp you must generate an API key from your MailChimp account and save it into the addon's settings of your WordPress website.

Generate an API Key from your MailChimp account - read here how:
Login into your WordPress dashboard and navigate to: Users -> Mailchimp and add the API key you've just generated, into the "MailChimp API Key" setting.

Press the "Save Changes" button.