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- Configuring User Model
http://code.tutsplus.com/tutorials/create-beautiful-administration-interfaces-with-active-admin--net-21729

- this stackoverflow discussion solve it 
http://stackoverflow.com/questions/8360787/rails-3-can-active-admin-use-an-existing-user-model

- better follow the documentation!
http://activeadmin.info/docs/0-installation.html