public void SetCamera( 
   Bookmark bookmark
)

//Set the extent of a map frame to a bookmark.

//Perform on the worker thread
await QueuedTask.Run(() =>
{
  //Reference MapFrame
  MapFrame mf_bk = layout.FindElement("Map Frame") as MapFrame;

  //Reference a bookmark that belongs to a map associated with the map frame
  Map m = mf_bk.Map;
  Bookmark bk = m.GetBookmarks().FirstOrDefault(item => item.Name.Equals("Lakes"));

  //Set the map frame extent using the bookmark
  mf_bk.SetCamera(bk);
});