After installing keycaptcha plugin please do the following:

1. Login to your site as administrator.
2. Go to System & Settings > Permissions & Access > Captcha setup.
3. Activate keyCAPTCHA plugin. After that you should get userID and Private key from keycaptcha site.
4. Register at https://keycaptcha.com;
5. In your account click the link "Add site"
6. Put all neccessary information about your site.
7. At the last step you can see Private key. So, copy it to the Captcha setup page in your concrete5 dashboard.