To use, select the tool. If the image has a measure or scale in it, click on both ends of it. A measuring line will appear between the two points. The ends of the line can be moved by clicking and dragging on the + at either end. The entire line can be moved by clicking and dragging on the line itself. Be sure to zoom in quite closely to make sure the ends of the measuring line are where you want them.
Check the length of the measuring line in the "Info" palette.