Follow this tutorial to access another mailbox in Outlook 2010
Note: You need full access rights to a mailbox before it can be added to your Outlook email account.
1. Open Outlook 2010 and click on the File tab
2. Under the Info tab, click the Account Settings button then select Account Settings… from the drop down menu
3. Click the Change button
4. Click the More Settings Button
6. Select the Advanced tab and click the Add button
7. Enter in the Add Mailbox field the email address or username of the account you want to access
8. Click Apply and OK. On the Change Account window click Next and then Finish
9. Exit the Change Account window. You should now see the new mailbox listed below your existing mailbox.