You should be able to add the value expression for the tooltip from the Values Grouping Editor -> Action tab. Select Jump to bookmark or Jump to URL and enter the expression in the Label field.
You can review the following examples shipped with the installation: ChartTypes_Combo.rdl; ChartTypes_Combo_Manhattan_Bar_Chart.rdl;
Hopefully this helps. Let us know if you have any questions or if you need assistance.
Nevron Support Team