From the comments on the plugin page, some Mac users have had issues with getting it to find their editing applications, and it is still in beta, so your YMMV.
What I did as a workaround was point the plugin at the
/usr/bin/open utility. This will open the field’s content in a document in whatever app you have as your default text-file opener — as
man open says,
The open command opens a file (or a directory or URL), just as if you had double-clicked the file’s icon. If no application name is specified, the default application as determined via LaunchServices is used to open the specified files.
Hopefully they will get this working properly soon.