First, there must be an ROI in the current recording. When one is present, the "Show Motion Vectors" icon will become available. You can click on this icon to add the display to your ROI(s):

The vectors have attributes which you can control by clicking on the drop-down arrow to the right of the icon. Generally, the vector display is controlled as a "group." You will see this menu:

Link to video showing how to add the Motion Vector arrows to your recording.