login to your FreeNAS GUI,
click Jails (1) then Add Jail (2)
in the Jail Name box (1) type the name you want to call your new Jail, I prefer not to use spaces, then click OK (2)
wait for the jail plugin to finish downloading
the highlighted items are your Jail Name and your IP address. At this point you already created a FreeNAS Jail.
update your Jail
click on your Jail (1) then click on the button with the black box with a C:/> (2)
it should bring this black box call Shell (highlighted)
type pkg update and press Enter
type pkg upgrade and press Enter
if asked to proceed type y and press Enter
now just sit back and let it finish downloading, installing and checking.