In Outlook go to the Calendar screen.
In here click on the Add Calendar button.
This will open the Global Address List (GAL)
In the GAL type in the name of the Mailbox/Calendar or Person, who's calendar you want to open and click ok. This will add the calendar to the folder list (for quick access next time).
You will first need permissions to the calendar in order to open it. If it is a persons calendar, they can give you permissions themselves to open it. If it is a shared/team calendar then permissions can be given by either the owner or IT.