Yes, when you add the add to cart button, it will add the button form to the page along with the behavior, so you need to add it outside of any existing forms.
What version of eCart do you have installed? are you using eCart 6? make sure you have the latest version installed that is available in your download center.
also, reset the workspace layout:
go to Window -> Work Space layout -> Reset '<your layout name>'
and Delete the Dreamweaver Cache file from the users configuration folder: