Creating Sample Point

[Krikor]

"To create a sample point, Ctrl-click on one of the two measurement rules of the image window and drag the mouse pointer. Two perpendicular guides appear. The sample point is where both guides intersect. You can see its coordinates in the lower left corner and the information bar of the image window. Release the mouse button."

This guidance is correct.
Are you actually clicking the ctrl key or the commmand key?
Considering your OS, the key should be command.