The installer currently can't be used on Mac, due to how MultiMC is an app instead of a folder. You can use the zip file to install it.
Visit https:/maven.aristois.net/manifest/(MC VER).zip?launcher=multimc, but replace (MC VER) with the Minecraft version. It will download a zip file containing the instance (Ex: 1.19.3-Aristois). Drag that folder into your instance folder, then the instance will appear and you can use it.