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: http://kb.mailchimp.com/accounts/management/about-api-keys
- Login into your WordPress dashboard and navigate to: Users -> WPUM Settings -> Extensions and add the API key you've just generated, into the "MailChimp API Key" setting.
- Press the "Save Changes" button.