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.
Copy the unzipped dictionary folder into the appropriate directory depending on your device or operating system:
/sdcard/koreader/data/dict/mnt/private/koreader/data/dictkoreader/data/dict.adds/koreader/data/dict/applications/koreader/data/dict$HOME/.config/koreader/data/dict$HOME/Library/Application Support/koreader/data/dictOpen KOReader on your device. The dictionary will be automatically detected and ready to use for text lookups.