MAMultipleOptionDescription

Not sure how to do this with Magritte. I have a model 'MYBuilding' which has many 'MYAmenity' objects. When showing the editor for building, I want the user to be able to select multiple amenities from the amenity collection in MYAmenity class>>collection, rather than creating new amenities for the building. This is the description I have for MYBuilding class>>descriptionAmenities descriptionAmenities ^ (MAMultipleOptionDescription auto: 'amenities' label: 'Amenities' priority: 40) options: MVAmenity collection; reference: MVAmenity descriptionTitle; beSorted; yourself. It's not working. I get an MNU on MVAmenity>>do: I'm guessing it has something to do with what I'm sending to the reference selector, since I have no idea what I'm supposed to have there. Thanks in advance. Chad