If I remember correctly, this adapter can only be used with an Doctor v64 CD Version. The adapter should work similar as the 64DD capture card. It enables booting from the expansion port.
If you want to play US games on a Japanese system (or the other way around), you only need a 4,5mm Nintendo gamebit.http://www.amazon.com/Screwdriver-Sec ... esis-Master/dp/B000F78NBQ
with this, you can open the N64 and simply remove the plastic underneath the slot.http://www.youtube.com/watch?v=QfZcZYBB40U
Besides the different stickers, there is no difference between an US and a JAP console. You can even use the Powersupply unit from any region (also PAL), as long as you use a converter.