johnwasser:
My guess is that you switch has a light that runs off the +/- power terminals. The other three terminals are "Common", "Normally Open" (connected to Common only while the button is pressed) and "Normally Closed" (connected to Common only while the button is not pressed).The picture of the terminal welder is too blurry to read the smaller text. What is that light blue-green box connected to the ground terminal? It appears to have a switch in it... And that switch connects one side of your momentary switch to the '+' side of your battery while the other side of your momentary switch is connected to the '-' side of the battery. If both switches close, something will fry.
Perhaps a pointer to the source of the diagram will help.
here's is the schematic. The video is only 20 seconds.
As shown in the schematic, one end of the momentary switch is connected to solenoid and the other one to the negative terminal of the battery ( This is the same momentary switch that will be connected to Arduino, too).