First of all, DNG converter 8.8 was the first version that supported your camera. That doesn't mean you are limited to that version. You can use any subsequent version of the DNG converter.
The DNG converter does not work on individual files. You will notice in the converter interface that it prompts you for the FOLDER that contains your raw images. You should select the FOLDER in the DNG converter, but don't open the folder and try to select individual DNG files because it won't work. Because of this nature, it would probably be best for you to move those images from the desktop into a folder, and then choose that folder for the DNG converter to work with.
To add what Jim suggested, there was a note for that camera that 8.8 was the preliminary version, but 9.0 was the main version. So download the latest version and make sure you save the converted dng files for version 8.1-8.5.
Many thanks guys. I would have been floundering around for ages with this ! Whilst it is normally all there somewhere, I do not find Adobe information or help easy to find, understand or use.
Thanks again for the expert advice.