Shared folders are typically used by several members of the same team for tasks, emails, etc.
In order to connect to a shared mailbox Aqua Mail will look for the Inbox, Sent Items, Deleted, Spam, etc. which may not always be available.
There is also no good way currently to support viewing a shared Calendar.
To add a shared mailbox go to Aqua Mail Settings>choose the exchange account>Accout Setup>Manual
Also, go to Aqua Mail Settings> Manage Accounts>Select your account>Options>Exchange options>Make sure that Sync all folders under Calendar is enabled