From the Folders view when you select a folder from the tree, on condition you have the necessary permissions, the "New folder" button will appear on the button bar.
To create a new folder you need to select the parent folder and then click "New folder".
The "New folder" button only appears in the folders for which the user has permissions to manage their structure.
The "New folder" button opens a window where you can enter the name of the folder to be created.
Clicking OK will create the new folder.
It is also possible to create folders with the options menu of the folders, by right clicking on the folder of the tree from which you will "hang" the new folder. Both in the folder tree and in any window in which the created folder structure is displayed.
Others ways to create folder can be found in the folder administration.