This one caught us by surprise - the bottom line is the 8.X technique for adding a Widget to the iNotes Mail outline no longer works.
It is summed up by this line in Technote 1627817 "In iNotes 9.0, the Widgets folder in the Mail outline is no longer supported. The documentation surrounding this feature remains in the iNotes 9.0 documentation but does not apply to iNotes 9.0." Go ahead, read it again ...
Instead of using the old technique of a single XML file, you need to follow the procedure outlined in Technote 1627817, but be warned the procedure is a rambling mess. It appears to be a cut and paste hack job that lacks flow or sufficient detail.