Viewing your folders in your computer's Outlook client can be somewhat different than viewing it using the Web version.   In the client version, you can generally double click the name of the folder or click on the menu arrow to the left to get it to display sub-folders.  Here is a screenshot from a computer-based Outlook client:

In  the Web version you are unable to double click on the folder name to expand it and the menu arrow looks somewhat different.  Here is a screenshot of the same account information taken from the web version of Outlook.  Notice the arrow circled in red.

In the web version, you must click on the menu arrow in order to expand the folder and see all of it's contents.  This is what it looks like after the menu arrow has been clicked.  Notice the arrow is now pointing upward and the folders it contains are shown.

One last thing to remember, the web version of Outlook can be slow.  If you click on a menu arrow and nothing happens right away, please be patient.  Sometimes it can take a few moments before it begins to list your folders - especially if you have a large number of them.