when installing or uninstalling an extension, you need to close dreamweaver first.
Close dreamweaver, then re install eCart and Delete the Dreamweaver Cache file from the users configuration folder:
then restart Dreamweaver.
Before you can use eCart, you will need to first define a dynamic site in Dreamweaver, then create a new PHP page and save it to the site you have defined.
see the "Defining a Dynamic Site in Dreamweaver" tutorial on the learning resources page for more details: