First, let me say that I have never built a PSX to PC converter, so I can't say whether or not those instructions are actually correct.
As far as the multi-meter goes, I think he meant you should use it to test the power supply if you are using the force feedback ability of the controller. In other words, make sure your power supply is really a 9Volt DC supply (test it w/ the multimeter) before connecting it to the psx pad.
Again, whether a 9volt supply is really what you should use, I don't know...but that's what the instructions on that page suggest.
Hope this helped...