I bought my MacBook Pro while living here in Japan. Naturally it arrived with a Japanese version keyboard. For the most part, this was not a problem and is even a help for the occasional Japanese language typing. I did run into problems however once i tried to type either an underscore or a backslash. For a while I got around this by using the Character Viewer built into OSX. I would just navigate the Character Viewer to "., Punctuation", then select the underscore and hit "Insert".
The Character Viewer option got old pretty fast, so I went in search of a better solution... and surprisingly found it fairly easily. If you hold down the "alt option" key on your MacBook keyboard and then hit the ¥ key, it will type a backslash. For the underscore, just hit the key that is immediately left of the RIGHT side shift key (it's the hiragana character for "ro" and there is a small dotted square just above the ろ character. In most apps, that will produce an underscore (I know for certain it does in OpenOffice and the OSX terminal app).
FYI: The aforementioned "dotted squares" relate to the position of a Japanese character (or part of a character) that you are typing. For English writing they serve no purpose.