This example turns on drop lines for chart group one in Chart1 and then sets their line style, weight, and color. The example should be run on a 2-D line chart that has one series.
With Charts("Chart1").ChartGroups(1)
.HasDropLines = True
With .DropLines.Border
.LineStyle = xlThin
.Weight = xlMedium
.ColorIndex = 3
End With
End With