To Create auto incremented field with ECO II you have to set its class attribute type to integer and its property PMapper to AutoInc (Write exactle like that or ECO should complain) then change the property SaveAction to DBAssign.
If you are using ECO III just set the type of your class attribute to AutoInc and set its SaveAction to DBAssign.
The SaveAction seted to DBAssign makes ECO reread the attribute after persisted to the DataBase.
The SaveActions are:
None: its a normal attribute.
Freeze: It cannot be modified after saved inside DB.
DBAssign: Reread the attribute after persisted to the Database.