The reason I asked which server language you were using was because eCart 5 only supports PHP. You will have to open a PHP page for eCart 5 to run.
As for the error you are getting now. With Dreamweaver and extension manager closed, try renaming the configuration folder referred to in this document:
This will have the effect of disabling all your extensions. Once you have renamed the configuration folder, open the extension manager and remove eCart. Try installing it again and testing it. Make sure you open a PHP page first.