I just installed collective.externaleditor, but there is problem. In the plugin settings page, it is written that "The users also have to enable this in their preferences". But there is no such settings in the preferences page.
When I try to open a file using external editor, I get this error message: "External Editor property in your preferences is not activated."
It depends on the version of Plone you are using. In Plone 5, we use plone.app.users >= 2.3, where the external editor preference has been removed.
You can put it back by adding a boolean field named ext_editor in your Member fields (go to Site Setup / Users and groups / Member fields), I hink it should work.