I have mediatomb in ubuntu. I have started it with "sudo service mediatomb start", and its process shows in ps.
But in the config I cant find where to specify the media directory. I searched the web too, with no luck.
AFAIK there isn't a way to specify your media path in the config file. You need to do it in the web interface. But first you need enable the web interface (and best put it in a known port).
You can replace 4000 with the actual port you want to use. Just make sure nothing else is using it.
Then go to http://localhost:4000/ in a browser, find your media path in the tree, and add it using the buttons on the right handside of the screen.