I will repeat my suggestion that if dealing with the command prompt is difficult, you might want to think twice about getting into ROM hacking. It's not as simple as finding a text file where you replace the words with new words.
But I'll try. At the moment, you have no idea where any text exists in the files on the UMD, so what we're trying to do is run a simple test to see if there is any Shift-JIS text anywhere in any of the files. So here's a step-by-step process.
1. Put SJIS_Dump.exe, SJIS_Dump.cpp and the files you're analysing in the same folder
2. In Explorer, right-click->New->Text Document
3. Rename the new text document "sjisdump.bat" (if you haven't turned on file extensions in Explorer... do it
4. Open sjisdump.bat in Notepad (right-click->Edit)
5. Paste this into it and save:
SJIS_Dump.exe "file" dump.txt 5 0
Make sure you change "file" to whatever the file is that you want to search, so if it's EBOOT.BIN then replace "file" with "EBOOT.BIN" (include the quotes)
6. Double-click sjisdump.bat
7. Read dump.txt to see what you found
There you are. If even this is too much then I highly recommend finding a competent hacker.
I'd try myself but as I said, I need to find the game.
EDIT: okay, I got a bit frustrated with all this and tracked down my own copy of the game.
So the first file I tackled was Diva2Script.cpk, since it has "script" in the title.
There's clearly some Shift-JIS text in there: for example, at $7FB097 there's 株式会社セガ, meaning Sega Corporation. Plus what looks like some staff credits. Next I tried Diva2Data.cpk, but the text file was 10MB and most of what I could see was junk data.
I'll have a look through the other files to see if something comes up, maybe there's more in the script file that I missed.
EDIT2: alas, we're unlucky. That script file is the only one with Shift-JIS text in it. A search of the entire ISO for リズム brings nothing, and that should appear somewhere. I'm currently doing a relative search to see if something comes up there, but I have my doubts. It's possible that it uses a different coding standard for some strange reason. Curiously, starting from $76800 in Diva2Data.cpk, there's a list of every possible Shift-JIS and ASCII character that the game could use, so there must surely be a reason for it to be there.
Hmm... looking through the list of Shift-JIS characters there, they come in sets. 32 characters, followed by 0D 0A, then another 32 characters. Maybe it's a lookup table, and each 0D 0A is a break that the game searches for. But I can't see why they would bother doing this when Shift-JIS takes two bytes and this system would also need two bytes for each character. Makes no sense. Hmm...
EDIT3: huh, whaddaya know, HMPD2nd got a translation already, and it looks almost the same as this one:http://www.romhacking.net/translations/2641/
Maybe looking at that will give me some clues...
See, I've never hacked the PSP before, so I had no idea that there was something specific about it... the EBOOT.BIN file is encrypted, and the PSP decrypts it. That's why searching for Shift-JIS text gave me nothing. But thankfully, the PPSSPP emulator has an option to dump the decrypted EBOOT.BIN after booting the game, so I did that, and all the text is there! I guess I should've gone with your initial suggestion to check out the EBOOT.BIN file, but I had to try patching the HMPD2nd translation before I realised what had happened.
As you can see, it was quite easy:
So all you need to do is find the Shift-JIS text in the game and change it to ASCII standard text, and it just works. You can type the ASCII text in any hex editor, but finding the Shift-JIS text will require the use of that tool I mentioned. Phew, I'm glad I figured this out.