By default, TileMill runs as a desktop-style application on Ubuntu, but it can also run as a system-wide service managed by upstart.
sudo start tilemill
sudo stop tilemill
sudo restart tilemill
/usr/share/mapbox
/var/log/tilemill
/etc/tilemill/tilemill.config
Once TileMill has started you can access the interface in your web browser at http://localhost:8889
.