How do I export/import diagrams so I don’t lose my work as I move between work locations?
You can simply share the UMR file. Alternatively you can export/import the XMI file, but that is only your domain model, not your diagram. You will need to recreate the diagrams by dragging and dropping your classes from your Class view outline back onto the diagram.
XMI is a format that only describes models that the UML tool community agrees on, no one has come up with a standard for the actual diagram (visual part) itself yet that everyone agrees on.