Hi @Adamh! May I extend the warmest of welcomes to our lovely community. I am sure you will enjoy it here.
The handling of devices is currently changing. In the stable version it should work as documented. In testing version we have a new device management system that will detect and auto configure some USB MIDI devices, e.g. Launchpad.
So we need to know what version of the Zynthian software you are running. This is most easily shown with a screenshot of the webconf console.
I think that this function isn’t working properly in the last stable version. After selecting trigger device channel selecting line disappear and one cannot select channel (selected trigger note on all channels on device start pad playback)