As Chris says, you can use "next control" to move through the dialog. When
you have said it once, and you reach the "look in" combo box, you can use
the "move down one" command to move through message subject, subject field
and message body, frequently used text fields etc to specify the criteria.
Saying "click find now" should press that button without going through the
whole dialog assuming you want to search in the current mail folder.
I actually think jaws can be scripted to speak a lot better in that dialog,
but it is not bad.