This example sets the chart type and then sets the marker type and interior color for the specified series.
Set c = ChartSpace1.Constants
ChartSpace1.Charts(0).Type = c.chChartTypeLineMarkers
ChartSpace1.Charts(0).SeriesCollection(0).Marker.Style = c.chMarkerStyleSquare
ChartSpace1.Charts(0).SeriesCollection(0).Interior.Color = "blue"