You must enable CORS in the S3 bucket that River4 runs from. Here's a copy of the configuration I used for my bucket.
You must be signed on to Twitter. We only use Twitter for the ID system, RiverBrowser won't post anything to Twitter, or access any of your messages or private data. The command to sign on is in the system menu, at the right end of the menu bar.
Choose My rivers in the Rivers menu. An attribute editor pops up, with all the rivers that are currently being displayed by RiverBrowser. You can add or delete items from the list, or change the names of any of the items. This is where you can add pointers to your own rivers.