Sorry for the confusion. This tileset was not made in Rpg Maker format, and it is also not stated on the page that this tileset was specifically made for Rpg Maker. What is stated is that this tileset pack includes a sample map created using Rpg Maker MV. Have you tried playtesting the sample map?
Edit: If you want to open the sample map you can try program called Tiled map editor https://www.mapeditor.org/ just extract Sample Map.zip. then go to folder Sample Map > maps then, inside you can open JSON file using Tiled. Hope that helps.