open the HistogramTransformation process, then click the check mark at the bottom right of the histogram process window, and then click on the view that you want to see the histogram for.
the check mark causes the process (or any process with a checkmark) to automatically switch it's attention to the view that you have highlighted (that is, the view that has a blue title bar and is frontmost on your desktop.)
the histogram tool shows you what the histogram is in the upper pane, and what it will become in the lower pane after adjusting the sliders below the lower pane.
rob