There is a couple of things that could be causing this issue.
Make sure that you have a WA_eCart folder in your site. And if you open any of the page, the link to the eCart folder is correct.
Make sure that you are using the same eCart version that the site was created with. for example if the site was created with eCart 3, then you will need eCart 3 to make your changes.
In the Server behaviors panel (Window -> Server Behaviors) do you see an eCart object listed?