before using eCart, you need to create a Dynamic Site Definition in Dreamweaver.
See the following tutorial for details on defining a dynamic site in Dreamweaver:
http://www.webassist.com/community/tutorials/view_tutorial.php?tid=93
Once you have defined a dynamic site, create a new file with the .php extension and save it to the Dreamweaver site.
you will then be able to add eCart to the site pages.