Install CB AntiSpam then within CB AntiSpam > Parameters > Captcha > Modes > reCaptcha to supply your keys. Then under Captcha > General set Mode to reCaptcha. Captcha usages should be reCaptcha ready at this point. You can also just set Mode to Joomla and it'll output whatever captcha you're using in Joomla. Now if you want captcha on registration simply set the antispam_captcha field within CB > Field Management to display on registration.