Hi Andie,
First remove all your custom fields added to the customer_h table and remove the fields also from configuration.
You have to select the business object in second popup, Suppose if you are adding field to the service orders( BUS2000116) you have to select service process as enhance BO object.
Similarly you are doing changes to activity business object you have to select Appointment from the above popup.
Once you select this you can select customer_h object for enhancement. You can proceed further and refer url given in previous threads.
Best Regards,
Dharmakasi.