To add additional mailboxes you first need to have the appropriate permissions on the server.
1. Click File tab | Info | Account Settings | Account Settings... The Account Settings dialog box will pop up
2. Select your default account and click Change... The Change Account dialog box will appear
3. Click on More Settings...
4. Select the Advanced tab and then click on Add
5. Enter the name of the additional mailbox into the Add mailbox field and click on OK. The additional mailbox has now been added
6. Click on OK.
7. Click on Next.
8. Click on Finish. The additional mailbox has now been added to the Navigation Pane.