It looks like the product isn’t really supported on the Mac.
As others have mentioned, you can simply take the micro SD card out from the top of the device. You can find a card reader at PoundLand or any good electronics shop. Plug the SD card into the reader, the reader into your Mac and you’ll have access to all the books on the card. You can delete them, rearrange them or add new ones that way. You don’t need any special software – the card will just show up as another disk drive.
If you were specifically told by staff that it would be supported on the Mac, you should be able to return it for a refund. Or contact Elonex Support and see if they can help.
All the best,