You can receive notification when you start Mavim that new transport databases have been put in the distribution centre (See Activate Distribution Centre). The distribution centre can be opened directly from this notification. You can enable this option via the system options. See Miscellaneous.
This is the notification you will, see:
If you click the message in the notification, the distribution centre will be opened. You can then import the new transport database(s). See Import Transport Database (*.mtdx).
