You can only add someone's calendar if he is in your team or the person has given you permission to his calendar.
follow these steps:
1. Right-click on the "Other Calendars"
2. On the menu click "Add Calendar"
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3. Choose from "Address Book" and the address book will show up.
4. On the Address Book make sure that you point the drop-down to the Global Address list and search for Matt McAdam.
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