//Must be on the QueuedTask.Run()
string terrainPath = @"d:\Data\Terrain\filegdb_Containing_A_Terrain.gdb";
var terrainURI = new Uri(terrainPath);
var terrainCP = new TerrainLayerCreationParams(terrainURI);
terrainCP.Name = "My Terrain Layer";
terrainCP.IsVisible = false;
//Create the layer to the terrain
var terrainLayer = LayerFactory.Instance.CreateLayer<TerrainLayer>(terrainCP, map);