The problem will be that your palm doesn't support unicode characters. Can you view Japanese characters using any other program on your palm (other than specialist software like PAdict)? If so then I think that you will need to alter the supermemo settings to use a unicode font instead of the standard western font. If not then you'll have to do something that involves putting unicode fonts on your palm and perhaps more than this.
I've only used supermemo on windows so I can't really offer any definitive advice I'm afraid.
I've only used supermemo on windows so I can't really offer any definitive advice I'm afraid.