Yes, take a look at your NFS log on the Ubuntu box (/var/log/daemon.log). It will usually tell you exactly why you are getting denied.
Are you using nfs-kernel-server?
Can you post what your /etc/exports looks like?
Have a look at http://kb.vmware.com/kb/1005948
Make sure you are using NFS v3.
Ubuntu is using NFS v4 is this OK?
As per the following thread only NSFv3 is currently supported.
http://communities.vmware.com/message/1286932?tstart=0
Cheers
Kevin
Thanks NFS is working now by using NFS v3