The current version of the LaTeX Equation Editor does not work terribly well out-of-the-box on Japanese OS X. However, some kind souls have sent the following tips which have allowed them to get the Equation Editor working on their machines. Perhaps it will work for you as well. (Many thanks to Taku Yamanaka, who first informed me of this trick.)
I found one quick solution for running LaTeX Equation Editor under Japanese OSX. After selecting Japanese language and relogging in, on Terminal, if I do: % setenv __CF_USER_TEXT_ENCODING 0x1F5:0:0 % open /Applications/LaTeX\ Equation\ Editor.app then the Equation Editor runs fine under Japanese language system.