Ok, the first thing you need to keep in mind is that when you connect to a Citadel server with IMAP, all your personal mailboxes are going to show up as subordinates of INBOX. INBOX is your Mail> room. This was done to maintain compatibility with some other software out there (particularly Kolab clients, which expect the groupware folders to be in places like 'INBOX/Calendar' etc.)
The next thing to be aware of is that Citadel honors the NAMESPACE extension to IMAP. That means when an IMAP server asks Citadel for namespaces, it's going to list the user's personal namespace (INBOX and its subordinates), other users' personal namespaces (none, at present), and the namespaces of public folders (in Citadel, that's all the floors on your system).
For some stupid reason, Evolution simply is not interested in giving you access to public folders. It just doesn't do public folders on an IMAP server (but it does when you're using the Exchange connector – go figure).
There is a workaround, though. Delete the account from Evolution completely, and create it again. When you configure the account, check the box that says 'Override server-supplied namespace,' and leave the namespace blank. After Evolution connects, you'll see the public folders.
→ in latest evolution versions this doesn't exist anymore, so there is actual no cheap way to view public floors. You might want to vote for this bug: http://bugzilla.gnome.org/show_bug.cgi?id=498475 or use this workaround. It seems as Evolution 2.30 will finally solve this.