Noob! I'm disappointed in you, I even gave the link to the manual where it explains how to do it!
"If the rysnc server requires password authentication, input --password-file=/PATHTO/FILENAME in the "Extra options" box, replacing /PATHTO/FILENAME with the appropriate path to the file containing the value of the password."
Three Hail Mary's in penance.
Arik - if the "module" isn't revealed at the iOmega end, it's almost certain to be the same name as the share you're syncing with. 'Module' is basically the rsync equivalent of a 'share', and not including it is likely why the connection to the iOmega is failing. When defining the rsync task on FreeNAS you must specify the module to connect to on the iOmega.
As an alternative plan of action, you could perhaps configure up the rsync task from the iOmega end? It's easy to get the necessary module, user and pass for the FreeNAS side.