Theoretically, this should work:
Qt Code: 
  1. bool OgreWidget::mousePressEvent( QMouseEvent* e )
  2. {
  3.     MarkerNode = mSceneMgr->getRootSceneNode()->createChildSceneNode();
  4.     //if (mTrayMgr->injectMouseDown(arg, id)) return true; //do you need that for something???
  5.     if( e->button() == Qt::LeftButton )
  6.     {
  7.         ent_marker = mSceneMgr->createEntity( Ogre::SceneManager:: PT_SPHERE);
  8.         MarkerNode->attachObject(ent_marker);
  9.         MarkerNode->setScale(0.1f, 0.1f, 0.1f);
  10.         Ogre::Real screenWidth = Ogre::Root::getSingleton().getAutoCreatedWindow()->getWidth();
  11.         Ogre::Real screenHeight = Ogre::Root::getSingleton().getAutoCreatedWindow()->getHeight();
  12.  
  13.         Ogre::Real offsetX = e->posF().x() / this->width();
  14.         Ogre::Real offsetY = e->posF().y() / this->height();
  15.         Ogre::Ray mouseRay = mCamera1->getCameraToViewportRay(offsetX, offsetY);
  16.         std::pair<bool, Ogre::Real> result = mouseRay.intersects(CubeNode->getAttachedObject (0) ->getBoundingBox());
  17.  
  18.         if(result.first)
  19.         {
  20.             Ogre:: Vector3 point = mouseRay.getPoint(result.second);
  21.             MarkerNode->setPosition(point);
  22.         }
  23.         break;
  24.     }
  25.  
  26.     QWidget::mousePressEvent( e );
  27. }
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