Are not really sure why you are getting the js warning about the object. It might clear itself up when you have the client side response validations applied to the page.
To apply the response validations to have it hide or show the element you will need to select the same element that you applied the client validations to, then go to the same client validations area and choose 'show/hide layer'. This will serve as a response to the client validation. When they work together it can be used to hide or show an element or area of the form based on weather or not a checkbox was checked or any number of other client validations that can trigger the response.