Note: The steps below are only required for IMAP accounts. They are *not* required for Exchange accounts.
1. Go to the Folder tab in the menu bar, click Update Folder List.
Sometimes it doesn't work and you have to quit Outlook and restart it.
If the above doesn't work for you, try these steps:
1. Again go to the Folder tab in the menu bar, click IMAP Folders.
2. Click Query.
3. In the Folders list, click the folder you want to subscribe to. Click Subscribe.
Now you should see new folders appear in the folder list.
And these new folders will automatically re-sync with your emails.