A simple trick to get Navit to center on your maps... Assuming you have your map files correctly specified in Navit.xml...

Using the internal Gui, click anywhere on your blank screen. Go to Actions and then Town. Type in the name of a town that should appear within your map data. Click on the town name once it fully appears in the list. This will bring up a menu of things you can do with that location. Click "View on Map".

You should be good to go...

For more information you can check out the following wiki page:

http://wiki.navit-project.org/index.php/OpenStreetMaps