Click on Outlook->Tools menu->Email account->View or change existing e-mail accounts->Microsoft Exchange Server, select Change->More settings->Advanced and add the other user name as the mailbox to open.

Change to Folder List view; you should now see the additional mailbox. Click on the "+" to see the subfolders (see below). If you get a permissions error, then you need to go back to the Owner's Outlook, right click on their "Mailbox - <owner's name>", click on Properties, click on Permissions, Add the delegate's name to the list and give them a Permission Level of Publishing Editor.