Here is a pinout that shows the location of pin #5:
That pin is connected to your house earth ground wiring via the third prong of the Command Base power cord.
To improve TMCC signal in weak areas (typically areas of dense tracks or where tracks cross over each other), you connect a wire (any gauge) to pin 5 and route around the problem area. Precise geometry is probably not important; you can experiment to see what works best.
If you have large areas of metal around the layout that are interfering with signal (wire mesh; bridges) you can connect the earth ground wire to that metal to improve signal. Do not connect the earth ground wire to layout common (outside rail)!
