When you need to access a non-default calendar in another user's mailbox, you need to add their mailbox to your profile. You can use this method with all versions of Outlook, including Outlook 2010.
Outlook 2010 supports multiple Exchange server mailboxes in a profile. If you need full access to a mailbox and know the password, you may want to open it as a separate mailbox. However, opening a delegated mailbox as a second account is not recommended. Delegates should use the method described on this page with Outlook 2010.
Type in the name of the person who is sharing their calendar. If you enter a partial name, the pick name dialog will come up.
The mailbox is now visible in your folder list and you can see the folders you have permission to see.
If permissions are set correctly on the shared folders and mailbox, only the folders that are shared will be visible in the folder list.
See How to view shared subfolders in an Exchange mailbox to earn how to set the permissions on the folder you want to share.