To open an editor for a folder’s external definitions:
Select a folder in a source list or browser view.
Select File > Open Externals for … (key equivalent ⇧⌘X) or click the Externals button in the app’s toolbar:
An externals editor will be opened for the selected folder. If an editor was already open for the folder then that editor is brought into the foreground and re-activated.
An empty document is displayed in the editor if the selected folder doesn’t have an svn:externals property.