I create a select all button in the Customize Mode window, and place it in a toolbar.
I go back and select a feature class in the table of contents.
I see the Select All button is grayed out.
ESRI customer service says this function is not working and that I must go back to what I was doing (and I should mention it here):
Right click on the feature class, in the dropdown, go to Selection, and in that dropdown, click on Select All.