When you first log in, the OWA displays only the most recently used folders. A “More” link appears below the folder list. Click that to see all your folders. The first set of most recently used folders will appear as “Favorites”, and the list of all folders below that, under your name. A scroll bar will appear next to the folder list, if necessary.
If a folder has sub-folders, the parent folder will have a “v” symbol next to it. Click that to see the sub-folders; the “v” turns into “^”. Click the “^” to hide the sub-folders again.