Viewing another persons inbox

Ask that person to verify your permissions on the top level of that mailbox. In order to add a mailbox to your own folder list, the root mailbox folder should have at least "Folder visible" permissions for your account.
Once that permissions has been set, restart Outlook on your computer and try to open the mailbox again.

 Exchange Server?  If yes, the same way as earlier versions of Outlook. 'File tab|Account Settings' click "Microsoft Exchange", click "Change", click the "More Settings" button, on the "Advanced" tab, add the mailboxes you wish to view to the "Open these additional mailboxes" pane.

src: http://answers.microsoft.com/en-us/office/forum/office_2010-outlook/viewing-another-persons-inbox/53873784-3a17-45c4-842c-e916b980a25f