Before attempting to add a new attribute to an Active Directory Object, here are a few steps to help ensure success:

1. Make sure that the attribute either already exists in the Active Directory forest or can schema-extend the forest, and verify that the attribute is correctly mapped to the class of the object
2. Verify that the account you will be using to add the new attribute has the proper permissions
3. Check the security settings of the object to ensure that you are allowed to set the new values of the new attribute
4. Check any replication constraints to make sure that all relevant sites are updated with the new attribute
5. Verify any applications that use the attribute to ensure that the new value you are setting is supported