The NFS server must be reachable on a network segment connected to a physical interface where the VMkernel has access. Use vmkping to check connectivity.
In out case, the NFS server was connected to a network adapter reserved for use by the service console, this is incorrect.
To fix the problem, under "Networking" click on "Properties" for the appropriate virtual switch and select "Add" | "VMKernel". You should now be able to add the NFS storage volume.
