By default, the folder list showing all your folders and sub-folders is displayed on the left side of your screen in Outlook Express. If a folder contains sub-folders, either the sub-folders will be displayed under their main folders, or a + sign will show next to the name of the main folder. Clicking on the + will display the sub-folders.

If the folder list isn't displayed, click on the View menu and click Layout. In the screen that comes up, click on "Folder List" to check it, then click OK.