Put the field in the Details section and then right-click on it and select the menu options Insert > Summary. If the 'Summary' option is grayed out, then this means that the field is a formula which already uses some type of summary formula and you can't do another summary on it.
I cover how to create summaries and give walk-throughs in chapter 3 of my Encyclopedia book. You can find out more about my books at
Amazon.com or reading the
Crystal Reports eBooks online.