First, you need FreeNAS installed on a server. Then you’ll need to turn on a service (under services) iTunes/DAAP. Config this by selecting your music directory and following the logic of the page. If you installed FreeNAS, this config shouldn’t be an issue.
==update Jan 1, 2014 ===== So FreeNAS is no longer FreeNAS, it is now called NAS4Free and some company has acquired the rights to call their bloated NAS “Freenas” and have the old FreeNAS.org website. So stick with NAS4Free because it will run on older, weaker hardware and have all these wonderfully mature and stable options like DAAP.
Second, go to the Droid market and download the free DAAP app. Configure it to point to your FreeNAS server and it will read your music library in a minute or so.
That should leave you good to go.