// Activate the tool
var documentBars = ((IDocument)ArcMap.Document).CommandBars;
var cmdItem = documentBars.Find(new UIDClass { Value = ThisAddIn.IDs.PolylineTool });
ArcMap.Application.CurrentTool = cmdItem;

// The tool's OnMouseDown event
protected override void OnMouseDown(MouseEventArgs arg)
{
    var rubberBand = new RubberLineClass();
    var geometry = rubberBand.TrackNew(ArcMap.Document.ActiveView.ScreenDisplay, new SimpleLineSymbolClass { Color = new RgbColorClass { Red = 255 } });
    geometry.SpatialReference = ArcMap.Document.FocusMap.SpatialReference;

    if (!geometry.IsEmpty)
    {
        var container = (IGraphicsContainer)ArcMap.Document.FocusMap;
        container.AddElement(new LineElementClass { Geometry = geometry }, 0);
    }
}