If the extension seems to be behaving oddly for you it might be a good idea to clear the cache to see if this helps. Here are the steps for this:
After clearing the cache open DW and open up only a single page that has a CSS Form Builder form on it. With only this page open select the form then choose to edit the contents or design.
If you have any further trouble with this let us know and post back with some screen shots if you can so that we have a visual reference for the issue.