Now that Ubuntu 15.04 has moved to systemd, Mediatomb no longer runs from the dash.

If you want to run it on a per user basis, open a terminal and enter

$ mediatomb

However, if as I do, you prefer to run it system wide, it's better to use systemd

$ systemctl start mediatomb

To run Mediatomb on bootup

$ systemctl enable mediatomb

Note that it now runs on port 50500, for whatever reason.

I've learnt a lot about systemd over the past couple of weeks, mainly from the Archlinux wiki. Here are a couple of links: