Add the root folder called "areyouahuman" in the root of your Silverstripe installation. Make sure to rename it to "areyouahuman" if it is not already named so.
In your site _config.php you can set the publisher and scoring keys like this:
AYAHField::$publisher_key = 'your publisher key';
AYAHField::$scoring_key = 'you scoring key';
If you want the Are You a Human field to be the automatic Spam Protection field, also add the following line to your _config.php:
SpamProtectorManager::set_spam_protector('AYAHProtector');
After you have configured your keys, add AYAHField to the forms you want validated via the "Are you a human"-service (www.areyouahuman.com). Something like this:
$formFields->push(new AYAHField());
You must have registered an account at www.areyouahuman.com to be able to use their service. It's free!