I am looking for a way to have a product that is free but the only why to get it is if you have a coupon. Any ideas?
Make the product cost a certain amount and then if the coupon is for the same amount they get the product for free.
Yes I agree with @thehpmc. Also you can in product filed 'Terms Text' write down that product is free so user will see it instead your price. Also I recommend edit your signup form and click 'configure' link on Coupon brick and make this field required.