You did it right. He's talking about earth grounding. Which is something you should do with the metal parts of your cabinet (coin door, monitor frame, control panel if it's metal). To earth ground, run a wire from a metal component to the bottom round prong of an electrical cord. Earth grounding helps protect electronic components from static discharge as well as helping to protect you from an electrical shock from a short circuit (keeps you from being the most convenient path to ground).