I'm replying to this old thread for the sake of those who may search the forum in the future.
I'm the author of the free RTEJava utility. RTEJava works on both Windows (95 and later) and Mac OS (8.1 and later, including Mac OS X).
I built RTEJava for myself to use on my Mac. RTEJava is not perfect, however RTEJava works!!!
RTEJava is a Java application, so it may work in some ways that are unexpected if you have used only native Windows or Mac OS applications in the past.
If RTEJava won't launch at all:

• Ensure that Java is installed correctly. Refer to the included READ_ME.HTM file located in the Documents subfolder.
• Mac OS 8 or 9 users need to ensure that Swing is installed correctly, too. Refer to the included READ_ME.HTM file.

If you encounter a problem when choosing the key file while decoding a rte file: - The name of the key file appears in the FileChooser dialog as a HINT only: You must navigate to the folder that contains the key file you want to use and click on it to actually choose the key file.

If you encounter any other problems while RTEJava is running: - Click the About button to get information about your OS and Java version, along with the RTEJava bug report e-mail address.

• Write an e-mail (English only, please), explicitly mentioning any error messages that RTEJava displayed, and the OS and Java version shown in the About box.
• Send the e-mail to the RTEJava bug report e-mail address shown in the About box.

Don't hesitate to use the RTEJava bug report e-mail address: I can't improve RTEJava if I don't know what problems folks are having!