It may be sufficient to just use standard File Server capabilities to serve media to another computer on the network.
However, if you have an XBOX 360, PS3, or other Home Theatre PC (e.g. XBMC), a UPnP media server is necessary. Helpfully, FreeNAS comes with FUPPES built-in.
To enable this: Go to Services -> UPnP
Tick Enable near the top right.
All the defaults should work.
Just add the appropriate directories for Database and Content (which can both be /mnt/name_of_HD for simplicity).
Click Save and Restart.
Any network connected UPnP media player should now be able to find your media.
If not, go to http://192.168.1.250:49152 (substitute 192.168.1.250 for the IP of your FreeDNS server, or just click on the link near the bottom of the UPnP settings page) and click Configuration from left-hand menu.
Under IP address or network interface name (e.g. eth0, wlan1, …)/HTTP port, the name of your ethernet connector should appear; just click on the Submit Query button below it. If this still doesn't work, manually add the IP address of your network media player at the bottom.