Our company is using an IMAP mail server with many shared folders (for example, all email going to info@mycompany.domain goes into a shared "info" folder which is visible to every employee).
RoundCube displays only the private folders, not the shared folders. They are not listed in the "Folders" tab in the settings page.
If you wish, I can set up a demo IMAP mail server having such shared folders to help you debugging this issue.
You can also set up an IMAP mail server with shared folders yourself using this howto:
http://www.debianhowto.de/doku.php/de:howtos:sarge:exim4_vexim2_courier_mail... http://www.courier-mta.org/imap/?README.sharedfolders.html
cheers, Arne