When users are having problems accessing public folders there are a few things to check. The first is to make sure that the user’s outlook is connected to the exchange servers. To check this go to the File menu and make sure “work offline” is not checked.
If the user’s Outlook is connected to the Exchange servers then it might be a permissions problem with the folder they are trying to access. Have an administrator verify the permissions to make sure the user has access to the folder.