The AddDataPoint method is not in the SimpleDataset3D class, rather it is in in the underlying SimpleDataset class, where it is assumed that the underlying z-value is the value of ImplicitZValue property. However, there should be an AddDataPoint method in the SimpleDataset3D class. It is simple enough to write external to the class though.
int AddDataPoint(SimpleDataset3D ds, Point3D p)
int n = ds.XData.Add(p.X);