This version of the editor only supports "split channel" mode (ie 1 MIDI channel per part - which is the factory default).
Connect your Volca Drum to your chosen MIDI interface.
Select that MIDI interface in the Midi Device Setup (the channel doesn't matter).
The patch will be sent to your Volca automatically when the interface is chosen.
Make your patch!
My preferred way to "save" your patch is by clicking "Create Sharable Patch Link" and saving the link somewhere (and don't forget to share it!). You can also save a stream of MIDI CC data which can be played back using a DAW etc. by clicking "Save Patch". You can also save the current state of the patch as a kit on your Volca.
If you have questions you can get hold of me on one of many social media platforms:
Oh, and if you want to create your own midi controller you can find the library I build to create this patch editor here: ccynthmata