when you create a from with form builder, an xml file is created in the webassist/forms folder with the form settings and is used if you try to edit the form.
if the XML file is deleted from the site, the edit wont work and will give the error you report.
without the XML File in the webassist/forms folder, form builder cannot be used to edit the form.
you will need to manually add the checkbox to the form using Dreamweaver design view.