When you are editing a form with form builder you need to select the form on the page, then in the property inspector you will have options to edit the form, design, or contents. To edit the form that is applied to the page choose to edit the design or the contents.
If all you are trying to do is add in validations to the form you may not need form builder to do so. You can apply spry validations one at a time by selecting the the field in the form and going to Insert > Spry then choose the spry validation type for the element you are working with. Once applied you can configure it in the property inspector.
You can also apply the WebAssist Server or Client side validations by going to Insert > WebAssist > CSS Form Builder and choosing server or client validations.