Ah the joys of NetInfo... ;-)
The /etc/exports isn't used in X. Instead it stores that information in a binary database called NetInfo. You use the NetInfo Manager.app in /Applications/Utilities to make the changes.
Now using NetInfo Manager is not for the faint of heart; it isn't very well documented and the metaphor takes some getting used. Basically it is laid out like a file system, with folders. These folders have properties/value pairs attached.
So, for exports:
1. Create a directory at the / level (it should appear alongside /machines, /users, etc.). Do this from Directory->New Subdirectory.
2. Select the new directory. You'll notice in the bottom pane that there is a Property of "name" with a value of "new_directory" . This defines what the value of the property "name" is. Double-click on "new_directory" to change it to "exports". Choose Save to update it.
3. Select the /exports folder and add a subdirectory under it.
4. Follow the same procedure from step 2 to change the name property to match the path of the exported directory (i.e. "/Users/Shared").
5. Now to define options for the export choose the "name" value of your export and choose Directory->New Property. This adds a "new_property" to the export.
6. Double click "new_property" and change the name to "opts". This (obviously) is where you'll define the specific export options you want.
7. Next you'll add the option values for your export. Select the "opts" property and choose Directory->New Value. This adds a value "new_value" in the right column across from "opts".
NOTE: Properties can have multiple values, as you'll see. They're like a stack or an array attached to a single property.
8. Double-click on "new_value" and change it to the option you want WITHOUT THE LEADING DASH. This is the same as exporting in /etc/export; however, mountd will add the '-' where necessary.
i.e. "opts" | "mapall=nobody"
9. To add another option, make sure you have the first option selected (in my example "mapall" and choose Directory->Append Value. Add as many as you need.
10. SAVE!!!
11. Ok, once you have your export defined you'll have to either reboot or drop to the command line and "sudo mountd" to launch the mount daemon. Check in /var/log/system.log to check for errors. "showmount -e" will let you know if you have any active exports.
There you go! Simple eh? ;-)
A simpler way is to go to
http://www.versiontracker.com and download NFS Manager.app. It provides a GUI for doing this, but it always helps to know what's going on.