Not to state the obvious, but you are not reading the post correctly which you linked to above. Excerpt from that posting:
First problem - keeping your power supplies where you want them to stay.
First thing to realize - ground is ground is ground. If you plug a 15V power supply into the wall, it's going to be feeding off the same voltage as your PS2 is. And in the end, the current is going to go right back into the same place - the wall. What's more, the black wire on your PlayStation controller is ground - and that ground goes right back into the PS2, where it feeds itself around a whole lot of electronics, but in the end goes... right back into the wall. Connecting both grounds won't hurt a thing. Now, a little more confusing is that if you hook up 15V worth of batteries, the ground will work exactly the same as if it's going into the wall - don't ask me about that. :3
The problem is your positive supplies. Those don't mix, as the lights work on about 14V, and your PlayStation board only works on 5V or so (standard for logic). So what do you have to do? Keep the one from going into the other. Now, if you know basic basic electronics, you'll know that there's a simple little tool made to do exactly this. It's called a diode, and its only function is to keep current flowing in one direction.
Ground is ground is ground cannot be overstated enough. All grounds can be linked together, as they all do lead to the same exact place. Wiring up lights to work on your controller can be tricky, especially if it's your first time doing it.
The diodes are used to protect the PS2 controller from being overloaded with too much current from the battery. The lights need to be connected to the battery for the correct amount of current to make them light up. The PS2 controller is connected to the microswitches, which do not require current to operate. These switches work on the simple NO or NC.
NO is "normally open" and NC is "normally closed". The 3 leads coming out of the microswitch are labeled as such, Ground, NO and NC. Your ground wire goes on the bottom terminal which is marked Ground. The NO or NC is used for the other wire, which either keeps the switch open or closed. When the switch is activated (the button is pushed), the switch changes from open to closed or vice versa.
Most switches are NO, meaning that the wires are not connected and hence no button or "circuit" is being created. If you wire up the micro to NC, then the button would be always pressed until pushed, making the circuit "open". Simple stuff once you understand that much.
Random's instructions are sound, just follow them until it makes sense. Hope that helps and good luck. | 
