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
Or if being used on a shared server add a system environment variable 
PGEO_DRIVER_TEMPLATE=DRIVER=Microsoft Access Driver (*.mdb, *.accdb);DBQ=%s