To display hidden fields do the following:
If the Customization Form is hidden, open it by right-clicking the control's header region and selecting Show Field List from the context menu.
The Customization Form that provides access to hidden fields will be displayed:
In the dropdown combobox, select the Pivot Table's area within which the field is to be displayed.
Click the Add To button:
Alternatively, you can drag-and-drop the field from the Customization Form onto the required area of the Pivot Table: