The code I found online, as shown below, which DLL should I refer to in order to use the AcMapMapMap class?
// Get the Map Object
AcMapMap currentMap = AcMapMap.GetCurrentMap();
Double viewScale = currentMap.GetViewScale();
ed.WriteMessage("ViewScale Before Change : " + viewScale.ToString());
// set the ViewScale
viewScale = (viewScale / 100);
currentMap.SetViewScale(viewScale);
ed.WriteMessage("ViewScale After Change : " + viewScale.ToString());