welcome to the Plugin Café community and thank you for reaching out to us. You did (almost) everything correctly with your first question, but I would nevertheless recommend having a look at out Forum Guidelines which line out the scope of support and how things do work here. One thing though, it is often best to provide executable example code for a problem.
About your Problem
??? text is intended an indicates an empty treeview, i.e., when its root contains no items. You are supposed to display here text as for example 'Drag documents into this field'. You can set the text with the method TreeViewFunctions.EmptyText().
When you continue to run into problems, I would have to ask you to share:
- Executable code which has been minimized.
- The version of Cinema 4D you are using.