From the Toolbar: Find the Mirror Tool icon on the Modify Toolbar (color coded green) and Click on the tool. The tool is now active and ready for use.

 
From the Shortcut Key: The default shortcut key for the point tool is the letter "M". Simply press the key and the tool will be activated.

 
From the Command Line: Type "mirror" into the Command Line, at the top of the notification window, and press the Enter/Return key.

This tool is a Stacked Tool; if there is no selection prior to the activation of this tool, the Selection Tool will become active. If entities were already selected when the  tool was activated then this step will be skipped.

1.        Select entities to be mirrored.

2.        Press Enter to finish the selection process.

Mirroring Axis: All selected entities get mirrored about this line. For ease of point selection, it is always useful to draw a mirror line first before activating this tool--then when specifying the axis (below) you can snap to the ends of the line.

1.         Move the mouse and click to specify the first point for the mirroring axis.

                       OR

        Move the mouse cursor to the Tool Property Tree and type in the first point coordinates into the "First Point" 3D Point Property Field found on the Tool Property Tree and press Enter to accept.

2.         Move the mouse and click to specify the first point for the mirroring axis.

                       OR

        Move the mouse cursor to the Tool Property Tree and type in the second point coordinates into the "Second Point" field and press Enter to accept.

Reset:

You have just mirrored one or more entities. The tool has now reset and you can either continue with other tools or use the mirror tool again.

Right Click: Brings up a Context Menu containing options for this tool.

Delete Original: Erases selected entities after they are mirrored.

Escape (Esc): Cancels current tool and activates the default tool (Selection Tool).

Space: Resets this tool.