how to participate ?
For Daily:
https://support.chess.com/customer/en/portal/articles/1444837-how-do-chess-com-online-tournaments-work-
For Live Swiss:
https://support.chess.com/customer/en/portal/articles/1444927-how-do-live-tournaments-work-where-can-i-join-one-
For Live Arena:
https://support.chess.com/customer/en/portal/articles/2862512-what-are-arena-tournaments-
I too have the same problem. Can't join a Tournament from the App. Or at least i dont know how to join using the App.
I joined using the website and now using the App to continue the games.
If you are on iOS, then it doesn't directly support tourneys in the app. Android does, though not Arena.
My understanding is that staff is testing them on an internal build of the app and it is going through the QA process on iOS.
You can sign up for tourneys on the website. Then play in the app. That works fine for Daily and some people do it on Live but I don't know how well it works in practice.