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.

API Key is availability:

Small Business Marketer Subscription

Corporate Marketer Subscription