Most likely it's server permissions
Setting up NFS is usually a bit fiddly.
I've got two home NAS machines, one of which has an NFS service which simply doesn't work from any client (Mac, Windows or Linux) and another which works but requires a great deal of messing around editing files and mapping user accounts. On this one I can also bypass all that by adding the ip address of the client to the table of permitted clients.
If you are able to specify a list of permitted IP addresses on your NFS server that may do the job.
There's also a third party utility called NFS manager that can make things a bit easier. I've not tried NFS mounts via disk utility, didn't even know it had that functionality.
Having said this, can I ask do you really need NFS? can you not share files via CIFS/SMB or if you're in an all Mac environment over AFP. As a rule I find NFS is slow a pain to set-up and insecure. Granted it's quicker than SMB on the Mac, but slower than AFP.
I did some benchmark testing recently and the results (sustained throughput for 42GB file transfer) were broadly as follows: (Server is BSD-based FreeNAS)
Mac AFP read 30MB/s write 30MB/s
Mac SMB read 12MB/s write 10MB/s
Mac NFS read 20MB/s write 20MB/s
Windows NFS read 20MB/s write 2MB/s (yes 2)
Windows CIFS read 90MB/s write 60MB/s
Curiously, Mac performance over SMB/CIFS to a Windows 7 desktop was much better at around 60MB/s read and 40MB/s write.
I'm told Apple have replaced the old SAMBA stack in Lion with a new optimised CIFS one, so hopefully OS X Lion will be able to deliver the same performance over the network as the Windows systems, but I've not tried it yet.