Step 1:  Highlight a tree-folder under which you like to add a new tree-folder.

Step 2:  Run New Folder command under Tree menu.  Or, Click a right mouse button on the folder to bring up a pop-up menu for Tree menu and then run New Folder command (as shown on right). 

Step 3:  This generates a new folder name "New Folder" under the folder you highlighted.  Edit the name of folder by simply typing a new name.

Step 4:  Edit the name of folder by simply typing a new name.

Step 5:  Press enter key to accept the changes.