I've been learning PivotTables recently and I'm just beginning Performing PivotTable Calculations. I'm trying to change the PivotTable Summary Calculation. I'm following my book's instructions step-by-step and all the pictures are the same, until my problem. All my Summarize By: options are grayed out (i.e. Sum, Count, Average, Max, Min, etc.)

Using Excel 2003

Here's what I'm doing:
Right-click on data field button
Choose Field Settings
Then I'm supposed to be able to select the summary calculation I want, but I can't. All the options are grayed out.

I've tried toggling everything I can find to toggle and nothing is working. Any ideas what would cause these to be greyed out? Is it a problem with my source data? Thanks for any help.