If you are asking about the agreement to Terms checkbox, we don't have any standard built-in way to add that; however, we are aware of a merchant who has devised their own way to require that before allowing the buyer to click Add to Cart -- see the products listed here:
http://commoncraft.com/
You can try to examine their page source to figure out on your own how they did it, or you could try contacting them for any advice they might be willing to share, but we cannot provide any specific advice for that and, indeed, we aren't even quite sure what they did there. :^)