Apologies but i haven't used CSSformbuilder or anything, I use this captcha in my contact form, and the code I used is based on the the php book by David Powers.
Anyway, the way i did it was to have a set of variables on form post, checking if the relevant fields were completed and if all variables were true, process the form and send the mail.
One of these check to see if the captcha field was completed, if it was, a check was then done to see if the code posted was the same as in the captcha field.
if all relevant variables were OK, the form was posted.
i will attach the page, (taking out personal info) which should give you the idea.
a link to the live page is contact.php
(best viewed in FF or chrome)