follow step 12 of the adobe troubleshooting guide to recreate the users configuration folder:
http://helpx.adobe.com/dreamweaver/kb/troubleshoot-javascript-errors-dreamweaver-cs4.html#main_user_config
then open the extension manager and uninstall eCart.
after eCart is uninstalled, use the install link in the extension manager to browse to the downloaded MXP file to install eCart 6.