[Nyororin]

The document she is talking about is something you will need only if you are already married, but have registered the marriage outside of Japan. It sounds like what she is looking at is not what you need to actually register a marriage, but instead what you need to get a spouse visa after marrying abroad.
I take it you need the more basic things for the first step, correct?

If you are getting married in Japan, it may actually be easier to get the documentation you need while in the country. You will need proof that you are eligible to marry in your home country. I would have no clue what that would be called if you were trying to get it before coming to Japan, but the embassies in Japan deal with it all the time and can get you the document you need in no time.

A quick question - are you planning to just get married in Japan? Or are there plans to live in Japan after getting married? Registering a marriage in Japan is quite simple - register yourself at the local public office with your passport and an ID photo, then file your marriage form with the proof of eligibility form. That is all there is to actually getting married officially...
However, getting a spouse visa to stay in Japan afterward is something quite different, of which filing your marriage papers is only the very first step.