When Network Manager starts up, it looks to see whether there are any "children" for the domain and dims the New button if there are. This prevents you from creating exports in a parent NetInfo domain.
This issue is caused by computer entries that have a 'serves' property. Creating computer entries, in Network Manager, automatically adds this property to the entry. There is no way not to add this property. Computers with the 'serves' property are called "children" of the NetInfo domain.
Solution
2. Select 'machines' in the second column.
3. Select each machine entry and look for a 'serves' property.
4. Select the 'serves' property.
5. Choose Delete from the Edit menu.
6. Save the changes.
Notes:
2. Do not remove the serves property for the 'broadcasthost' or 'localhost' entries.