The installer should be creating that file, I'll log a bug that it is not being added.
in the mean time, here is a copy of that file, you will need t locate the users configuration fodler, see the following tutorial for details:
http://www.webassist.com/tutorials/Common-installation-issues
in the configuration folder go to :
Configuration/shared/WebAssist/
if there is not a folder named eCart, create it and add the attached JS file to it.