I've written back and forth to Panasonic a few times. At first they hadn't heard of SD-Jukebox lol but I pointed them to the Japanese page and they have been in touch with Japan. They have confirmed that V5 is the most recent English version available and they are going to get back to me about whether people can still buy it or not.

For me V2.4 works with my HP computer which has an internal card reader that is SD Secure compatible so I'm sticking with that for the moment. I'll let you know what more news I can get.