Unfortunately unless the owner of the calendar gives you "reviewer" or higher access to their calendar you will not be able to add it to Outlook. The feature of adding a calendar with the default permissions of free/busy only works on systems running the Windows operating system.
https://support.microsoft.com/en-us/help/2876443/user-with-free-busy-permission-can-t-open-another-calendar-in-outlook-for-mac
As a workaround you can use the scheduler and add the users' calendar you want to view free/busy info for and it will be displayed (this is also an option on Windows systems).
You can also use Outlook online, aka webmail, which is almost as fully featured as the Outlook client.