Follow the Zabbix install instructions here
The Zabbix agent probably won't work and complain about missing libraries.
To fix, open a shell session on FreeNAS and create two symlinks:
1. ln -s /lib/libkvm.so.7 /lib/libkvm.so.5
2. ln -s /usr/local/lib/libiconv.so.2 /usr/local/lib/libiconv.so.3
Then you should be able to start the Zabbix Agent