Is it just me or does this not seem to work?
In model first i dragged in a simple table and created and enum (via convert to enum). Now i've tried with and wothout creating the enum items in the dialog, but if i tick the reference external type i get:-
"Schema specified is not valid. Errors: No corresponding object layer type could be found for the conceptual type 'ControlPanelDevModel.EventType'."