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"

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.