This is for people using Exchange but who manage additional mailboxes (other than their own).
First, you need to submit a help request to add your account to list of allowed account having access to the mailbox. Once permission has been granted, use the following instructions to add the second (or third or tenth) mailbox.
http://www.groovypost.com/howto/add-additional-mailbox-in-outlook-2013/
Outlook 2010 is very similar with only aesthetic differences.