Designs in OpenBuildings Energy Simulator are saved as DGN files within your Workset folder. However, double-clicking on these DGN's will not open the project in OpenBuildings Energy Simulator.
This is because DGN's are file types associated with OpenBuildings Designer - double clicking on the DGN will open the graphical part of your project only within OpenBuildings Designer. This can be easily distinguished by examining the Ribbon Bar.
OpenBuildings Designer:
OpenBuildings Energy Simulator:
To open your OpenBuildings Energy Simulator projects, please see this article.