# boot from Ubuntu LiveCD
# install xz-utils package
sudo apt-get update
sudo apt-get install xz-utils
# download
wget http://iso.cdn.freenas.org/9.1.1/RELEASE/x64/FreeNAS-9.1.1-RELEASE-x64.img.xz -O /tmp/FreeNAS-RELEASE-x64.img.xz
# install to USB pen
xzcat /tmp/FreeNAS-RELEASE-x64.img.xz | dd of=/dev/sde bs=64k
sync
reboot
# Links
http://www.freenas.org/