From My understanding this feature was dropped by cPanel when they issued the new courrier IMAP mailserver. I think you are referring to logging into webamail with your cPanel username and password > click on Horde.

This used to show every address you had under the one login.

cPanel says this functionality was a side effect of the old mail server and was not supposed to be a "feature" but after alot of complaints it sounds like they plan to re-add it soon. We will see.