This FAQ answers the following questions:
When you insert a form object in your page using one of the 'Insert | Form
Objects | ...' menu options, then DW inserts the object without specifying
the ID attribute for it.
When you rename the form object using the Properties
panel, then the ID attribute is added. But if you leave the name of
the form object unchanged, then there will be no element ID defined for the
form
object.
However, the extension dialog can only find form objects on the page
if they have a proper, unique element ID assigned to them.
So if a specific
form element on your page does not show up in the list in the extension dialog,
then make sure it has an element ID assigned to
it.