Not sure if the problem is resolved. Adding my part of discovery.
In the FreeNas AdminGUI, try to "Maproot User" and "Maproot Group" to "root" and "wheel" respectively. I was able to get ESXi 5.1 to access FreeNAS-8.3.0 exported NFS datastore after I did both mappings. Installed a Ubuntu VM with the NFS datastore.
[root@freenas] ~# cat /etc/exports
/mnt/freenas_vol1 -alldirs -maproot=root:wheel -nework 192.168.1.0/24