Start Outlook and go to Tools > Email Accounts...
Step 2
Choose 'View or change existing e-mail accounts' and click 'Next'.
Highlight the Microsoft Exchange Server account, then click Change...
Click on 'More Settings...'
Bring the 'Advanced' tab to the front.
Click 'Add...'
Enter the name of the mailbox you want access to and press OK. It will try to match the name you enter to an existing account name.
Now close the previous windows by clicking Next and then finish. Your new mailbox will appear in the list of Mail Folders.