The API key can be found in the account settings.

Administration->Account Settings:

It is important to note that the WP plugin does not require the API key unless you are using dynamic blocks and/or web forms. Neither of these features are available in the Email Marketer  nor the Call Tracking products. Thus there is no API key displayed for these package types.

Note: The API Key is only available in these packages:
-Small Business Marketer Subscription
-Corporate Marketer Subscription