When an app is published you cannot add or delete master items. From the online help:
When you are working with an unpublished app, you can create master dimensions so that they can be reused. Users of a published app will have access to the master dimensions, but will not be able to modify them.
If a user must be able to add a master item then you need to duplicate the app and make the user the owner. The user can then modify the master items and publish the app.