Adding a mailbox to a Microsoft 365 account and Outlook 2013

Go to http://portal.office.com
Login as an Administrator
click the drop down arrow to the left of "users" on the left-hand pane
click Active Users
Check box the user's whose name you are wanting to add a mailbox to
on the right hand side click Edit
click mailbox permission on the left
click more mailbox permissions at the bottom of the screen
type in the mailbox you want to add to the user

Go to the user's Outlook 2013 on their pc
File
Account Settings, account settings again
Email tab, Change, More settings, Advanced tab, Add
add the mailbox you just added permissions to in 365 portal

good to go

Right click the inbox of the newly added mailbox and add to favorites to make the mailbox show at the top of the outlook account

UPDATE  4/17/15--------------
Another way to add the mailbox would be to:
close any open Outlook sessions
go to the Windows Start Menu
type regedit and press enter to run it
expand the HKEY_CURRENT_USER
expand Software
expand Microsoft
expand Office
expand 15.0 (or whatever version you are trying to add the mailbox to)
highlight AutoDiscover and delete all entries below Default
Close the regedit
open the control panel
Mail (32-bit)
E-mail Accounts
Click New
add from here