I created a new ArcMap Add-in based on your code and it worked just fine - the code was able to add a raster to the ActiveView in ArcMap. See below:
protected override void OnClick()
{
string dir = @"C:\Temp\RasterFolder\"; // The path to to directory containing the Tiff file
string varNh = @"Clipped_DEM_Israel.tif"; // Name of the Tiff File
Type factoryType = Type.GetTypeFromProgID("esriDataSourcesRaster.RasterWorkspaceFactory");
IWorkspaceFactory wsFactory = (IWorkspaceFactory)Activator.CreateInstance(factoryType);
IRasterWorkspace2 rasterWS = (IRasterWorkspace2)wsFactory.OpenFromFile(dir, 0);
IRasterDataset rds = rasterWS.OpenRasterDataset(varNh);
IRasterLayer rasterLayer = new RasterLayerClass();
rasterLayer.CreateFromDataset(rds);
IMap mapa = ArcMap.Document.FocusMap;
mapa.AddLayer(rasterLayer);
IActiveView activeView = mapa as IActiveView;
activeView.Refresh();
}
I performed the test using ArcGIS 10.3.What version of ArcGIS were you using?