How can I get TidyCal to sync with iCloud calendar?

I got Tidycal which says it syncs with Apple calendar, but I can not get it to work. It needs me today that an event is busy but I don't have this option.

Has anyone successfully used Tidycal? How did you get it to work?

Tidycal aren't being very helpful and told me to ask Apple.

[Re-Titled by Moderator]

Original Title: Tidycal and icloud calendar