Problems with OVERLAY by OGRE

I have problems making use of OGRE overlays:

playState.h :

 #include <OgreOverlaySystem.h>
    #include <OgreOverlayElement.h>
    #include <OgreOverlayManager.h>
    #include <OgreFontManager.h>
    #include <OgreTextAreaOverlayElement.h>

    using namespace Ogre;

    Ogre::OverlayManager* _overlayManager;
    void createOverlay(); //Definicion de la funcion

playState.cpp

void PlayState::createOverlay() {
      Overlay *pMyOverlayLogo = (Overlay*) OverlayManager::getSingleton().getByName("Info");
      pMyOverlayLogo->show();
}

I do not understand why it explodes when I do the last line:

pMyOverlayLogo->show();

Could you show me an example of using Overlay?