Pete
In Pd 16.5 the help file says "PowerDesigner models materialized query tables as views with a stereotype of <<Materialized query table>>." Try it - the value appears in the drop-down stereotype list in the View properties. Select the stereotype and bingo, it all changes 
. You can't change the stereotype back again in the object properties, but you can do it in a list of objects.
The help text you were reading is obviously out of date.