By using this statement you can change your screen point to ArcMap Ipoint.
public override void OnMouseDown(int Button, int Shift, int X, int Y)
{
try
{
IMxDocument pDoc = m_application.Document as IMxDocument;
IMap map = pDoc.FocusMap;
IActiveView activeView = pDoc.ActiveView;
IPoint pnt = activeView.ScreenDisplay.DisplayTransformation.ToMapPoint(X, Y);
}
catch (Exception ex)
{
}
}