Editor ed = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument.Editor;

        [CommandMethod( " ZoomCenter ")]
         public  void ZoomCenter()
        {
            AcMapMap map = AcMapMap.GetCurrentMap();
            MgEnvelope mapExtent = map.GetMapExtent();

             double centerX = mapExtent.LowerLeftCoordinate.X + mapExtent.Width /  2;
             double centerY = mapExtent.LowerLeftCoordinate.Y + mapExtent.Height /  2;

            ed.WriteMessage( " center: "+centerX.ToString()+ " , "+centerY.ToString()+ " \n ");

            Point3d centerPt;
            PromptPointOptions ppo =  new PromptPointOptions( " Click on map to zoom center: ");
            PromptPointResult ppr = ed.GetPoint(ppo);
             if (ppr.Status == PromptStatus.OK)
            {
                centerPt = ppr.Value;

                MgEnvelope newExtent =  new MgEnvelope(centerPt.X - mapExtent.Width /  2,
                                                                    centerPt.Y - mapExtent.Height /  2,
                                                                    centerPt.X + mapExtent.Width /  2,
                                                                    centerPt.Y + mapExtent.Height /  2);

                map.ZoomToExtent(newExtent);
            }

        }