There are several steps to using the recaptcha library.
Wrap the responder in a <recaptcha>
Wrap the responder in an <alloy> if it is not already, to enable conversations and database access
Call validate-recaptcha from the validate slot of the action
Put the chloe tag <recaptcha/> inside a form tag in the template served by your action

There is an example web app using recaptcha support:
USING: furnace.recaptcha.example http.server ; <recaptcha-app> main-responder set-global 8080 httpd