Since I just wasted like an hour trying to figure this out, figured I might as well post.
I am running freenas 9.10, and like everyone else, was having problems. I could access either a CIFS or NFS share via an fstab command, but that command would have to be made to every single /etc/fstab file on each node of the cluster.
when I made used the add Storage->NFS option from the gui, the default path was incorrect (/mnt/pve/<storage_ID>, and while the ISO's would mount to the default folder path (shown in gui), they were not visible from the gui whenever I tried to make a new container or VM (ISO).
pvesm scan nfs 10.10.10.2 #(freenas address)
/mnt/Darkside/config/ISO 10.10.10.14,10.10.10.13,10.10.10.12,10.10.10.11 #(allowed hosts in NFS share configuration)
add in the storage with correct content type
pvesm add nfs NFS-iso --server 10.10.10.2 --path /var/lib/vz/template/iso/ --export /mnt/Darkside/config/ISO --content images,iso
verify storage was added:
Now, from the gui, the iso files will show up as "local" for all nodes in the cluster.