Support for Radiance in OpenStudio has always been spotty, at best. Illuminance maps are one of many Radiance-related objects that were never properly supported in the app. You can only create and modify illuminance maps in the plugin. ASSUming the map is created properly and linked to a zone, etc, it will export correctly to Radiance format when the Radiance Measure runs. You just can't view it or its properties in the app.