Is this an email sub folder you want to sync, as well as the inbox?
Select the user from Manage Users > Edit User > Default Configuration (at the bottom, under Messaging Configuration) > Email (tab) > (scroll down to:) Redirection Settings
Select 'Selected Folders'. The will then show you the folder list for this user, of which you can select the folders you want to sync.
The default is to sync 'Inbox only'.
Lee