First you need a
download package for the Kobo version. Then you can open the download package in your zip/compression program and find the "koreader" folder. With your device connected via USB cable, you can then just Drag&Drop the folder into the /mnt/onboard/.adds/ directory. The next time you start your device KSM should see that the folder is in /.adds, and add a Start koreader menu item, under the Nickel one.
You can do the same for the Pbchess package to add the vlasovsoft folder, for Coolreader and games & Utilities.
Luck;
Ken