Use the Measure tool to measure distances on the map. Each time you click the map, a new point is added. If you click the map three times you will end up with two line segments - one from the first point to the second point, and one from the second point to the third.
You can add as many segments as you like. As you move your mouse away from the last point, the segment length changes in the Segment text box. The next time you click the map the new segment length is added to the total length in the Total text box.
Use the Set Units tool to set units for measuring distances, buffering, and for the scalebar.
The Total distance of all of the line segments appears in the Total text box.
If you are done measuring, click another tool on the toolbar.
The Set Units panel appears below the map display area.