You would need to apply this client validation to the page manually after the form is created. Once you have the form on the page you would first select the checkbox that you would like to trigger the hiding or showing. Next go to Tag > Behaviors > WA Validation Toolkit > Show/Hide layer.
This is how you would apply the show hide validation to the element. Before you do this though you need to make sure that you have the required validation applied to the checkbox, this required validation is what will be used to trigger the show/hide behavior.