Click the widget menu (…) on the widget title bar, and click the Add Folder menu option.
To add a subfolder, click the target folder to open it. Then click the widget
menu (…) on the widget title bar, and click the Add Folder menu option.
Enter the name of the folder.
Click Add.
Optionally, you can add a folder by right-clicking an existing
folder and selecting Copy.
Place the cursor within the Menu widget (except within an existing link or folder),
right-click, select Add Folder, and select and
Paste.