Apparently this integration has been deprecated , and been added to HA’s core. Where in the UI can you “see” your ISY device?
It’s just part of Core, so you go to Configuration->Integrations and add it from there, or use YAML configuration: Universal Devices ISY994 - Home Assistant. Once you give it the IP of your ISY994 and credentials, a bunch of devices and entities will be created to represent the devices in your ISY.