If you use ChartDataClicked event you can get all that you need from the second parameter.
UltraChart1_ChartDataClicked(ByVal sender As Object, ByVal e As Infragistics.UltraChart.Shared.Events.ChartDataEventArgs) Handles UltraChart1.ChartDataClicked
You can try looking at:
http://help.infragistics.com/Help/NetAdvantage/NET/2008.3/CLR3.5/html/Chart_Drilldown.html