The proper use of /usr/local/ is a little advanced for me as well. When I edited /etc/rc.conf, with the startup script in /usr/local/etc/rc.d/, the urbackup_srv service starts and stops properly.
Another thing that would be good, though it probably isn't necessary, is to create a urbackup user on the FreeNAS system, and set the numeric userids on that account and the urbackup account inside the jail, to match. That way, you can have the dataset owned by that user, and have fairly restrictive permissions for it if you like. That would also show the urbackup process(es) under that user in top and other system monitoring tools. To do this, within the jail, run
Code:
# pw usermod urbackup -u 936
# pw usermod urbackup -g 936
# pw groupmod urbackup -g 936
# chown -R urbackup:urbackup /usr/local/var/urbackup
Then, using the FreeNAS web GUI, create a new user account. Set the user ID to 936 (a number I chose mostly at random; I'm certainly open to suggestions of a better one), the name to urbackup, the full name to Urbackup User (or whatever you want), and check the box to disable password login. Save the user.
Then go to storage, find the urbackup dataset, and edit the permissions. Change them to be owned by the urbackup user and group, and check the box to apply them recursively.
Then restart the urbackup_srv service inside the jail, and you should be set.