Jeff,
I’ve never used the DMX DUO box, but from what I can tell the firmware should be included in the download package of the ONYX software. Is this being used with a PC or to extend outputs of a console? Can you verify which ONYX 4.2 you are trying with? If a PC can you check your PC windows version.
Here is a link describing compatibility and drivers, if that doesn’t help. Maybe @Jingles can help. He is supporting some of the older hardware / Light Jockey. And lastly, it could be possible that it is at the end of life for the unit.
Hope this helps (what little it is),
Watson