You need to open up the Server Validations server behavior and add the Like Entry validation to compare the entered value to the correct captcha value.
To add the code manually, you can add this after line 33 on confirm7_cart.php.
$WAFV_Errors .= WAValidateLE((strtolower(isset($_POST["Security_Code_1"])?$_POST["Security_Code_1"]:"")) . "",((isset($_SESSION["captcha_Security_Code_1"]))?strtolower($_SESSION["captcha_Security_Code_1"]):"") . "",true,10);