Once you create your own package "MyOwnPackage " why would you go to option select existing package ? Select Existing Package is an option before 8.0 to create model in standard package (Over layering ) which is now not possible. So if you want to create extensions in future, select your project from solution explorer (Project model property set to your own Model [MyOwnPackage ]) right click on the object for which you want extension and hit create extension button. For classes , table method codes you have to create manually.
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