Select an existing Topic/Sub-folder and select New Sub-folder from the Edit menu, or right click on an existing Topic/Sub-folder in the Presentation Layout pane and select New Sub-folder from the drop-down menu.
When a Topic is selected, the new Sub-folder will be added as the first Sub-folder below the selected Topic.
When a Sub-folder is selected, the new Sub-folder will be added below the selected Sub-folder.
Enter a name for the newly added Sub-folder.
You can add Assets under the Sub-folder or move the Sub-folder up or down as necessary.