You can't really use /etc/fstab like a regular unix, everything in Mac OS X is done using the arguably wonderful NetInfo?. To mount an NFS share in a certain place under MacOSX, and make sure it turns up every boot, do the following:
|opts||-P -s -b|
Save this, go into a terminal and type sudo mount -a, and your share should appear in the directory you specified.
You might want to type man mount_nfs to learn more about the opts field.
|name||directory to export|
|clients||client ips (leave blank if you want to limit to your entire LAN)|
|opts||maproot=root networ=192.168.0.0 mask = 255.255.255.0|