I'm not sure whether this is a question about property grids or editors, but I'll go by the organization of the samples.
I added this to my UserControl.Resources, and I don't see my PropertyGridItem changing:
<propgrid:PropertyEditorsModifier x:Key="{x:Static propgrid:BuiltinEditors.PropertyEditorsModifierKey}">
<propgrid:PropertyEditorsModifierActionAdd>
<propgrideditors:EnumPropertyEditor UseDescriptionAttributes="True" />
</propgrid:PropertyEditorsModifierActionAdd>
</propgrid:PropertyEditorsModifier>
I also tried adding this to my PropertyGrid, with a similar lack of effect:
<propgrid:PropertyGrid.PropertyEditors>
<propgrideditors:EnumPropertyEditor UseDescriptionAttributes="True" />
</propgrid:PropertyGrid.PropertyEditors>
What am I missing?