Right-click the parent folder in the folder pane - New Subfolder

There is more than one method:

If you want to add a new folder as a top level folder into the mail account.

• Right click on mail account name folder in Folder Pane and select 'New Folder'

Small window opens.

• Enter name of folder you want to create.
• Create as a subfolder of: mail account name folder
• Click on 'Create Folder'

If you already have a top level folder and require a folder within it as a sun folder.

• Right click on the desired top level folder and select 'New Subfolder'

Small window opens.

• Enter name of folder you want to create.
• Create as a subfolder of: Folder name on mail account name
• Click on 'Create Folder'

Alternative via 'Menu app icon'

• Menu app icon > New > Folder
• Enter name of folder you want to create.
• Create as a subfolder of: Choose the folder or mail account as stated above, but note: window will open set as default on whatever folder you were focused on at the time, so you might need to change it using the drop down.
• Click on 'Create Folder'

If this is in an imap account, make sure you are subscribed to see it and also check Account Settings folder synchronisation settings are as required. Right click on new folder and select 'properties' to ensure all settings are ok.