The wonderful world of the Japanese eshop:
Go to the eshop and click on the button of an orange card with a plus sign.
Hit the button that shows the mastercard logo and says Visa.
I don't know what the next screen is, but it has two boxes and wants a number. It's not the security code, I put in the last two numbers of the security code last time (I thought that was what it wanted) and it would not accept any other number but the number I previously put, so remember this number! This screen does not show up in the English menu (it goes right to point amounts). Hit okay after putting in a number.
Next point values will show up. It won't let me hit 5000 again. :/ That's interesting to know.
Pick your points.
Next is a screen showing credit card logos, hit Visa.
(I'm not completing this next step. I don't want more points)
Now you will see a screen with a bunch of boxes for your credit card info (this screen is in the English eshop)
Long box- Credit card number
Next are two boxes with /20 in between them. This is the expiration date.
The next box is the security code. Once everything is complete, hit Okay. I believe that is the last step.
Edit: I may add pictures to this later today with the English and Japanese 3ds's side by side for comparison. It's already Sunday. Dx Good night!