You are in the forum right now but the window in your screen shot is the chat area here on Meta and you can’t add channels because it is not your forum. On your forum you will have admin capabilities.
Are you talking about here on Meta or on your own forum? There are no public channels here on Meta so you won’t find any to join. You can only use it to message individuals.
You can configure public channels if you choose to – see below for how we use it internally with public channels.
