Just noticed that the error message displayed, when an entity object type does not have an assigned reference mode, states "Error 2 Entity type 'EOT' in model 'Base' requires a preferred reference scheme."
In the properties window for this entity object type, there is a "RefMode" combo box.
I've heard that the subject of reference modes and value types are part of internal discussions at Neumont. Whatever the outcome, it would be best to settle on one term. Personally, I think "scheme" make more sense than "mode" in this context, but consistency within the tool and throughout the documentation is probably more important. By any name, the concept is important and took me a while to understand. You don't want to frustrate people new to ORM.