I wrote this text based on the words of the plugin developer (he probably misunderstood something)
A QR code is needed to join an existing chat.
In the official client, click on the chat name and select smth like "create invitation QR" This creates a QR code with a link. You can enter the link in Miranda (click the protocol icon in the status bar -> "Add a chat using the QR code").
But to communicate with the network, you need to have an account registered to your own email and specify the settings from this email in the Miranda protocol settings. If you registered via chatmail, you will not be able to connect Miranda to this account.