View "Report Data"

The "Report Data" pane is the one that lets you edit:

If you ever close it, it can be a hell of a job trying to find it again.

Here's the trick:

In the "Design" view, click anywhere on the report canvas outside of the report. i.e. on the background of the canvas on which the report is being designed.

Then, in the view menu, the option "Report Data" will be shown. Or click Ctrl+Alt+D.