Bring up the Chart Properties (Right Click the Chart object and select properties)

Now click the Edit button for the Values section and go to the Point Labels tab

Uncheck Auto to get percentages on outside (the inside square of 9 put labels in Pie Chart, the other are on the outside) and check the Show point labels box

Now the formula to enter into the Data label text box is:

=Sum(Value) / Sum(Value, "ChartDataset")

In this case the formula is:

=Sum(Fields!TotalDue.Value) / Sum(Fields!TotalDue.Value, "pRepSalesOrdersByMonth")

And the Label Style button opens up the label formatting properties

 

 

And this is the final product