This plugin adds a ReCaptcha in the ad creation from. In the plugin options, you can set your public and private key and choose a theme for your captcha.
To access the options, you need to go to the plugin options page. Click on the submenu link 'Module Manager' of the menu 'Extensions', then, click on the module title and click on the 'options' tab.
To generate a private and public key, go to this address: https://www.google.com/recaptcha/admin/create
You'll need a google account to create this keys.