public Object execute(ExecutionEvent event) throws ExecutionException {
	System.out.println("trying to open Graphiti Editor");
		
	File f = new File("C:\\...\\myDiagram.diagram");
        
        IPath ipath = new Path(f.getAbsolutePath());
        IFileStore fileLocation = EFS.getLocalFileSystem().getStore(ipath);
        FileStoreEditorInput input = new FileStoreEditorInput(
                                    fileLocation);
        
        IWorkbenchPage page = PlatformUI.getWorkbench().getActiveWorkbenchWindow().getActivePage();
        try {
              page.openEditor(input, "org.eclipse.graphiti.ui.editor.DiagramEditor");
              
        } catch (PartInitException e) {
               e.printStackTrace();
        }		 
		return null;
}

public Object execute(ExecutionEvent event) throws ExecutionException {
	IWorkbenchPage page = PlatformUI.getWorkbench().getActiveWorkbenchWindow().getActivePage();
        
        URI fileURI = URI.createFileURI("C:\\...\\myDiagram.diagram");
        URIEditorInput editorInput = new URIEditorInput(fileURI);
        
        try {
              page.openEditor(editorInput, 
            		  "org.eclipse.graphiti.ui.editor.DiagramEditor");
        } catch (PartInitException e) {
               e.printStackTrace();
        }
	return null;
}