To use reCaptcha you need an google account. You get an public and a private key, which you have to insert in the admin config of the reCaptcha SobiPro field.
To install this extension you need to use the SobiPro installer and not the Joomla! installer!
Only missing option to let only guests enter captcha (disable it for registered users).
Thank you for the rate. I added your request to the issue list.