Question: how to open mdb file in qgis?
Answer: For QGIS 3: In QGIS Settings panel -> Options | System | Environment add the following two variables: variable name: PGEO_DRIVER_TEMPLATE. value: DRIVER=Microsoft Access Driver (*.mdb, *.accdb);DBQ=%s. ... 
Now you should be able to start QGIS and simply drag the geodatabase mdb file onto the QGIS workspace and it will open.