After downloading and installing the app, right-click on the movie you want to re-rate in iTunes and select Show in Finder from the context menu. In the Finder, right-click on the movie file, select Open With, and choose MetaX.app from the list.
In MetaX, you will see a drop-down menu for Rating. Change it to whatever you want, then click on the Write button on the toolbar. (Note that, depending on the size of the movie file, you may have to select its Preferences -> General -> Enable Support for Large Files first.)
MetaX will proceed to rewrite the rating tag (and any other metadata you changed); this may take a while. When you go back to iTunes, the movie's rating should be updated.

