If you're doing this through the designer:
Select your column layer
Go to the ChartTypeAppearance property
Find ChartText collection
Edit the collection and add a new item to it
Set Column and Row properties to -2
Set Visible property to true

or in code:
ColumnChartAppearance appearance = new ColumnChartAppearance();
ChartTextAppearance chartText = new ChartTextAppearance();
chartText.Column = chartText.Row = -2;
chartText.ItemFormatString = "<DATA_VALUE:0.##>";
chartText.Visible = true;
appearance.ChartText.Add(chartText);
myLayer.ChartTypeAppearance = appearance;