The current beta of the workflow editor (v0.9.3) is now part of the latest Bitrise CLI release (v1.5.4), which means that you no longer need to install the plugin manually! If you install the Bitrise CLI 1.5.4 or newer you can start to use the workflow-editor right away!
When updates are available for the workflow editor, you’ll get notified when you start the editor, and you can update the editor with: bitrise plugin update workflow-editor