I updated home assist and then the integration appeared. Everything is working now.

When I go to integrations as per the guide:
"To integrate your Serial, Ethernet (LAN) or MQTT MySensors Gateway, go to Configuration >> Integrations in the UI, click the button with + sign and from the list of integration select MySensors."

There is no MySensors to select in the integrations. I have Home Assistant OS 5.9 running on a raspberry pi.