Download the dictionary file to your computer (pick the StarDict format, it should be named something like dict-XX-YY.zip).
Extract the archive to obtain the dictionary folder.
Connect your BOOX device to your computer with a USB cable.
Paste the unzipped folder into the storage/dicts directory on your BOOX.
Safely eject the device, and unplug the USB cable.
Open the built-in dictionary app, tap the options icon at the top right, click Preferred Dictionary, select your dictionary, and confirm with the tick icon.