What is the text of the error message? I can't troubleshoot an error without knowing what the error says.
after installing the extension in the extension 2manager, when you start Dreamweaver, it should launch the webAssist Installer to complete the installation If the WebAssist installer does not launch automatically, open a PHP page to force it to start. After the WebAssist installer completes, make sure you restart Dreamweaver.
"opening a layout/html page to edit"
eCart cannot be applied to a static .html page. If you have a static page open, eCart won't be able to work, you need to have a php page open in dreamweaver to use eCart.
before stating with eCart, the steps you should take are:
Define a Dynamic Site within Dreamweaver:
create a new php page in your site to add eCart behaviors to, and have that page open.