1) first you have to create a Recaptch account here:
http://www.google.com/recaptcha/captcha2) Go to your K2, than choose Parametrs icon , you will see K2 Parameters window.
Then choose Comments tabs, and Enable reCaptcha in comments [Yes],
3) And then "Advanced tab" and "Anti-spam Settings" there enter the keys and enable captcha
Save & Close
4) You can also go to : Plug-in Manager -> "Captcha - ReCaptcha" --> insert your keys, this for all joomla forms.
p.s.
By the way, read also my "old" post: http://gavick.com/magazine/how-to-use-r ... la-25.html