Make sure you're running Antinote 2.0 or later to use extensions.
~/Library/Application Support/Antinote/Extensions/ (you can also choose a custom folder)Any extension you put in the extensions folder can be loaded, which makes it easy to install custom extensions or ones shared by others. See Managing Extensions & API Keys for everything you can do once an extension is installed.
Tip: You can open your extensions folder any time from Settings > Extensions > Open Extensions Folder.