Multiple and Join gateways do not have a user interface because the engine runs them without the involvement of a user. You cannot process Multiple and Join gateways.
You can search for instances sitting in a Multiple or a Join gateway, or define a view that includes them. To view the instance details select the instance in the search result or in the view. If the instance is waiting for the engine to process it you can add notes and attachments to it. If the engine is processing the instance adding a note or an attachment causes an error.
You can view information about the processing of a Multiple or a Join gateway, in the audit trail.