I found the answer: To access another users “Inbox”, you simply right-click on your mailbox in the navigation pane in the left pane. You will see "add shared folder".
Specify the alias, username, or e-mail address of the user for which you want to open the Inbox. When clicking “OK”, you will now have access to the Inbox of the specified user via the navigation pane.
The interesting thing about this method is that this will be remembered until you right-click the other user’s Inbox and select “Remove from view”. Even next time you open your mailbox using OWA 2013 in Office 365.
If you get an error about permission, just go to the account you want to appear, double-click and go to mailbox delegation and assign "Full Permissions" to the account you want to add on. Now you can click on the drop-down menu and you can access the added mailbox.