|Mex Reference Manual|
If the Media Explorer user has the standard GNOME home directory
etc.), Media Explorer should automatically "see" any content added to
those directories and present it to the user.
However, if using a non-standard directory layout, or if you want to access files outside the default directories, you need to configure Tracker (which provides the content indexing service) from the command line with:
Under the Locations tab, add the appropriate directories to the Recursively Indexed directory list.
Now Tracker should find and index content in those directories, making them accessible via Media Explorer.
To find out more, see this section about how Media Explorer discovers content.