The video did not work but this seems to be a quick fix.
When you apply the multi/sub-object material to an object, the sub objects must then be defined with the mesh. In both Editable Meshes and Polies, there is a section (when either the Element or Polygon selection is enabled) called Material IDs.
So what you need to do is select and define what material number the selected polygons use. You can do this using either the Element or Polygon selection.
In a sense, the main material (multi/sub) is kind of like an index with more stuff inside. You then link the 3d object to the main material and then tell the polygons what material inside to use.