Yes, the MXP file for eCart 6 installs a base set of files, 2MB sounds about right. when you start dreamweaver, it should start the webAssist Installer which will downlload the rest of the files and install them.
Any time you are installing a new version of an extension, you should always uninstall the previous version from the extension manager first. Installing the new version over the perevious version will cause issues like you are having.
follow step 12 of the Adobe troubleshooting guide to recreate the users configuration folder:
then uninstall eCart from the extension manger and install it again.
when you open Dreamweaver, it should start the webAssist Installer to finish the installation process.