The fields available for each report are listed in two groupings: Attributes and Metrics (numerical performance data). When you select a report, the default fields will be selected for you (listed at the right). You can add or remove fields from this list as desired, as well as reset to the default selections.
Note that the display name for each field is shown, consistent with the web interface, and they are listed in alphabetical order. The list of available fields varies depending on the report your selected.
Select from the list at the left and click the [ >> ] button to add the field name to the selected list at the right. To reorder the list of fields, simply drag and drop them in the list at the right.