In design view you should select the form tag, there is a blue border around the CSS Form Builder forms, you should select this. When you do the property inspector should have the options for you to edit the form (preset), edit the contents, or edit the design.
You will want to edit the contents of the form, then choose to add a new group to the form, when it is added go to the bottom of the interface and for the category choose Security, then for type choose Security also.
This should add the captcha and security question, you can then edit the group if you want to remove the security question and leave just the captcha there.