what version of eCart is installed?
Make sure to install the most recent version, 6.0.5.
Uninstall the previous version of eCart from the extension manager, then install the newer 6.0.5 version.
after the install is complete in the extension manager, open DW, and a php page to start the WebAssist installer which will finish installing the eCart files. after the WebAssist Installer completes, don't forget to restart DW.