public void SetCamera( 
   Envelope extent
)

{
  //Note: Must be on the QueuedTask
  //Reference a map page
  var mpage = activePresentationView.Presentation.GetPage(4) as MapPresentationPage;

  //Set the map frame extent based on the new camera's X,Y, Scale and heading values
  Camera cam = new Camera(329997.6648, 6248553.1457, 2403605.8968, 24);
  mpage.SetCamera(cam);
}

{
  //Note: Must be on the QueuedTask
  //Reference map page
  var mpage = activePresentationView.Presentation.GetPage(4) as MapPresentationPage;

  //Reference map and layer
  MapProjectItem mp = Project.Current.FindItem("Page name") as MapProjectItem;
  Map map = mp.GetMap();
  FeatureLayer lyr = map.FindLayers("GreatLakes").First() as FeatureLayer;

  //Set the map frame extent to all features in the layer
  mpage.SetCamera(lyr);
}