You need to edit the main index.html (header and footer), which is located at the top of the tree. You must have attempted to edit the individual message (aka CONTENTS) instead.
The Language Pack Designer that is highlighted in the video is only for MWG version 6.x
It cannot be used for MWG 7.x.
There is a built-in editor in the GUI of version 7 that has roughly the same functionality.