The following article will assist the user in opening a shared mailbox in OWA provided the user has been given access to another user's mailbox.
1. Log into Outlook Web App at http://na02.msexchangeoutlook.com
2. In the upper right hand corner select the drop-down arrow next to your display name to pull up the "Open Other Mailbox" window.
3. Type in the first few letters of the display name of the mailbox you wish to open. Once you have located the mailbox select it and click Open. If the mailbox does not open, confirm you have the proper permissions.