Download the dictionary file to your computer (it should be named something like vivlio-XX-YY.zip).
Extract the archive to obtain the reader.dict-XX-YY.dict2 and reader.dict-XX-YY.dict2.idx files.
Plug the Vivlio One into your computer with a USB cable.
Paste both files into the dictionary folder on the Vivlio One (do not create subfolders).
Safely eject the Vivlio One, and unplug the USB cable.