1. Main menu>Edit>Administer websites, pick site
2. Users and Roles, pick role, click Edit role settings
3. go to Editing and check 'Allow users to edit web page source in external
application'
When your users opens (using the Choose btn next to addressbar) a .js or .xml
file in Contribute (tested in CS3) just click the 'Edit btn' and Contribute
will open the file in the editor you selected.
How to select the editors:
1. Main menu>Edit>Preferences
2. go to File Editors.
If this does not work in your version maybe you can use Main
menu>File>Actions>Edit page source in external application.