Using reCAPTCHA
Step 1
Head to http://www.google.com/recaptcha and sign up for a public/private key.
Step 2
In your ModX install insert/update the appropriate system settings for
formit.recaptcha_public_key, Your reCaptcha public key.
formit.recaptcha_private_key, Your reCaptcha private key.
formit.recaptcha_use_ssl, Whether or not to use SSL for reCaptcha requests. Defaults to false.
See formIt documentation for more information.
Step 3
Add "recaptcha" to your list of hooks in the form snippet.
//place recapture hook in the hooks array $o_form->setHooks(array('spam','recaptcha','email','redirect'));
Step 4
Add the reCaptcha element to your form.
$o_fe_captcha = new FormItBuilder_elementReCaptcha('Verification Code');
Customising reCaptcha (Version 1.1.1 and over)
You can add a JSON configuration string as described here https://developers.google.com/recaptcha/docs/customization, by setting options like so.
$o_fe_captcha->setJsonConfig('{"theme":"red", "lang":"fr", "tabindex":"2"}');