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:
The First Number Not Nameable in Under Ten Words 