It is recommended for you to install the mods you generate with this tool using the latest version of Mods of Mistria Installer.
Navigate to the Fields of Mistria folder where your game executable is, and if you don't already have a mods folder in there, create one.
Then, download and unzip this release into Fields of Mistria/mods. You will see a manifest.json and a fiddle folder.
This fiddle folder is the folder where you'll dump the customMonsters.json file you generate using the website.
Don't forget to install the mod using MOMI! Enjoy :)