Can I ask what are you trying to edit in that field. All the built in fields are managed. The only thing you can change in Created by field is searchable property to yes or no.
You can change some of the properties of these managed field for e.g max lenth in text fields, required level in most of them. You can delete them if they are not system fields.
Created By is a built in lookup field, you should not be changing it.
I hope this helps.
Amreek singh Senior CRM Consultant CDC Praxa Sydney,Australia http://mscrmshop.blogspot.com/