.po file name and location for the theme translation

I have created Japanese translation .po files for my theme name "MyTheme" (for Drupal 5.x and 6.x).
For Drupal 5.x, I named it "MyTheme.ja.po" and placed it under ./po subdirectory of the theme directory. I realized that I need to manually import the translation using the locale --> import function. It worked.

For Drupal6.x, I reserched a bit and found that ./translations subdirectory will be automatically looked in by Drupal. So I created ./translations subdirectory under the theme directory, then named the .po flle as "MyTheme.ja.po" and place it there. However, this po file is not automatically loaded by Drupal.
I tried several name variations such as

theme-MyTheme.ja.po
theme-MyTheme-6.x.ja.po (because theme directory name is MyTheme-6.x, although the theme name is MyTheme).
MyTheme-6.x.ja.po

None of them worked. Am I missing something?
Thank you for your help in advance.