An example of using Mirror is shown below. As in Plane Cut, the 3D transformation widget is used to position the cutting plane. See the Transform page for details on this widget. Alternately, you can left-click-drag to draw a 2D line, the mirror plane will be positioned to pass through this line (again, just like the Plane Cut tool).
When doing a Mirror, there are two possible mirror directions, one on either side of the cutting plane. Use the short, thick blue arrow widget (circled in red below-left) to toggle between these two planes.