If you look in the back of the machine you will see green wires going various places. The power block, the transformer, the switch in the back of the machine, the monitor, the speaker, the control panel and even the control panel latches.
You need a wire that is connected to the metal part of that button you speak of (soldered or with a terminal lug) and tie it into the green ground wires anywhere along the path.
That will keep you from getting shocked ONLY if you have the ground prong on your electrical plug that goes into the wall.
If you have a multi-meter, put one end of the meter on the third (ground) prong of the electrical plug and the other side to the monitor chassis (or any where else that has the green wire going to it). If you have continuity you are good to go, if you do not, or the ground prong is missing, replace that sucker ASAP.
NOTE: I mentioned the cables being green in color but that is only the original wiring that is green. It could be any color. Most likely green though.