Step 1: Log into your MailChimp Account and Find the API Key
Log into your MailChimp account, click your profile name on the top right corner of the page, and choose Account from the drop-down menu.
Click the Extras menu and choose API keys from the drop-down list.
Copy an existing API key or click the Create A Key button.
Step 2: Enter the API key and Select a Subscription List
Enter the API key to the plugin then click the Apply button. After the key is saved, select a list from the MailChimp list.