Do you have the PowerStore files in the root of your DW site? If not you can get this error. This is because eCart looks for the cart object by checking the WA_eCart folder in the root of the site. If the cart file cannot be found you will get this error.
If your files are not in the root of your DW site you should define a new site and specify the folder that your PowerStore is in as the root of the site.