As of version 4.7 of Mailchimp for WordPress it is no longer possible to add Google reCAPTCHA to your forms. Don’t worry though – you probably do not need it anyway.
Mailchimp for WordPress comes with built-in spam protection which should stop most bots from using your sign-up methods. If you are actually experiencing spam then we recommend the (free) WP Bruiser plugin to stop it.
You can read more about this in our help article on preventing spam sign-ups