Artigos Java Centralizando Dialogs e Messages no Eclipse
No lugar do new Shell() coloque getScreenCentredShell()

MessageDialog.openInfo(new Shell(), “E um teste Message Dialog.”);

/**
 * All dialogs and messages will be passed with the centered shell.
 * @return Shell
 */
public static Shell getScreenCentredShell() {
  final Display display = PlatformUI.getWorkbench().getDisplay();
  final Shell centreShell = new Shell(display);
  final Point size = centreShell.computeSize(-1, -1);
  final Rectangle screen = display.getMonitors()[0].getBounds();
  centreShell.setBounds((screen.width-size.x)/2, (screen.height-size.y)/2, size.x, size.y);
  return centreShell;
}