When viewing Outlook from within the Mail view, you cannot see the public folders you have access to, although you can see a folder, which looks like it would be the parent folder of the public or shared folders.
If the type of the public folders or shared folders is something other than email (for instance a calendar or contacts folder) then such folders are not shown when viewing Outlook in Mail mode.
To see other types of public or shared folders, open the appropriate view in Outlook, i.e. click on Calendars or Contacts in the shortcut bar. Alternatively, select Go | Folder List from the Outlook menu and you will be able to see all public and shared folders that you have access to.