First hour question - sorry! Add a Sonoff Thing

If you still haven’t installed the binding, then that’s another issue. This binding, it appears, hasn’t been added to the marketplace or the official list, which is why you didn’t find it in the bindings page on the UI. To install additional bindings such as these you need to have a complete jar file and place that jar file in the /addons directory in the OH configuration directory. It should be automatically detected.

In this case, it looks like the jar files are available on the forum thread for the project: