Comment Spam Pack - Custom Anti Spam not blocking spam

Hello - I've added the Comment Spam Pack to my site but the Custom Anti Spam plugin isn't working at all. Tested on IE, FF and Chrome - both logged in and logged out - it allows me to post a comment without putting anything in the captcha form. Even if I type a random message in the form, it's still posting.

Can someone confirm that this plugin works as expected and maybe point me in the direction as to what I'm doing wrong?