In User Profile, I was unable to modify any user property mapping due to missing Data Source field dropdown. There was no question of untrusted domain as I am using only one active directory and SharePoint was able to import from that. I finally managed to fix these by following these steps:
- Goto to Profile and Import Settings
- Click View Import Connections
- Click on Existing Import Connection and select edit
- In Authentication Information section, make sure the account is SharePoint Admin Domain account and not the default account
- Click Ok and you should now able to see mapping data source drop down
