Download the dictionary file to your computer (it should be named something like dicthtml-XX-YY.zip). Do not unzip it. Note: Pay attention if you are using Safari, as it may auto-unzip the file in the background.
Plug your Kobo e-reader into your computer using a USB cable and tap Connect on the e-reader screen.
Paste the dicthtml-XX-YY.zip file into the .kobo/custom-dict folder on your Kobo drive (you may need to create the custom-dict folder if it does not exist).
Safely eject the Kobo from your operating system, then unplug the USB cable. The dictionary will be ready to use.