A change to the XML Document structure was patched on the old MMX code-base and ported into Teiid Designer code-base that may explain this error. Also, EMF has been upgraded since Designer 6.3 release, so who know's what's causing this.

1) Import Books.xsd, and BooksDatatypes.xsd into Designer
2) Create XML DOcument model from Books and make sure you select the bookSetMixed schema node
3) After creation, select the "Choice" node in the XML Document tree and try to right-click select "Edit" action.
4) Get the following exception

The string resource '_UI_XmlEntityHolder_entities_feature' could not be locatedThe string resource '_UI_XmlEntityHolder_entities_feature' could not be located
at com.metamatrix.modeler.internal.core.ModelEditorImpl.executeAsTransaction(ModelEditorImpl.java:549)
at com.metamatrix.modeler.internal.core.ModelEditorImpl.getNewChildCommands(ModelEditorImpl.java:369)
at com.metamatrix.modeler.ui.actions.ModelerActionService.getInsertChildMenu(ModelerActionService.java:810)
at com.metamatrix.modeler.ui.actions.ModelerActionService.contributeToContextMenu(ModelerActionService.java:224)
at com.metamatrix.modeler.mapping.ui.editor.DocumentTreeViewer.fillContextMenu(DocumentTreeViewer.java:181)
at com.metamatrix.modeler.mapping.ui.editor.DocumentTreeViewer$1.menuAboutToShow(DocumentTreeViewer.java:83)
at org.eclipse.jface.action.MenuManager.fireAboutToShow(MenuManager.java:338)
at org.eclipse.jface.action.MenuManager.handleAboutToShow(MenuManager.java:469)
at org.eclipse.jface.action.MenuManager.access$1(MenuManager.java:465)
at org.eclipse.jface.action.MenuManager$2.menuShown(MenuManager.java:491)
at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:241)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1258)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1282)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1263)
at org.eclipse.swt.widgets.Menu._setVisible(Menu.java:199)
at org.eclipse.swt.widgets.Display.runPopups(Display.java:3593)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3168)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2629)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2593)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2427)
at org.eclipse.ui.internal.Workbench$7.run(Workbench.java:670)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:663)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:115)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:369)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:619)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:574)
at org.eclipse.equinox.launcher.Main.run(Main.java:1407)
at org.eclipse.equinox.launcher.Main.main(Main.java:1383)
Caused by: java.util.MissingResourceException: The string resource '_UI_XmlEntityHolder_entities_feature' could not be located
at org.eclipse.emf.common.util.DelegatingResourceLocator.delegatedGetString(DelegatingResourceLocator.java:487)
at org.eclipse.emf.common.util.DelegatingResourceLocator.getString(DelegatingResourceLocator.java:392)
at org.eclipse.emf.common.util.DelegatingResourceLocator.getString(DelegatingResourceLocator.java:366)
at org.eclipse.emf.edit.provider.ItemProviderAdapter.getFeatureText(ItemProviderAdapter.java:1844)
at org.eclipse.emf.edit.provider.ItemProviderAdapter.getCreateChildText(ItemProviderAdapter.java:1660)
at org.eclipse.emf.edit.command.CreateChildCommand.<init>(CreateChildCommand.java:220)
at org.eclipse.emf.edit.provider.ItemProviderAdapter.createCreateChildCommand(ItemProviderAdapter.java:1195)
at org.eclipse.emf.edit.provider.ItemProviderAdapter.createCreateChildCommand(ItemProviderAdapter.java:1179)
at org.eclipse.emf.edit.provider.ItemProviderAdapter.createCommand(ItemProviderAdapter.java:999)
at org.eclipse.emf.edit.domain.AdapterFactoryEditingDomain.createCommand(AdapterFactoryEditingDomain.java:464)
at com.metamatrix.modeler.internal.core.container.ContainerEditingDomain.createCommand(ContainerEditingDomain.java:110)
at com.metamatrix.modeler.internal.core.ModelEditorImpl.createCommands(ModelEditorImpl.java:3487)
at com.metamatrix.modeler.internal.core.ModelEditorImpl$1.run(ModelEditorImpl.java:365)
at com.metamatrix.modeler.internal.core.ModelEditorImpl.executeAsTransaction(ModelEditorImpl.java:541)
... 38 more