Adding Marketing Calendar to your Outlook Calendar

Add the Calendar to 'favorites' (sic):

To view the Public Folder Calendar alongside your own Open 'Calendars' in Outlook:
Expand 'other calendars' and click on the box next to the calendar you want:
Now the Public Folder Calendar can be viewed with my own:

Add the Calendar to 'favorites' (sic):
Click on the 'folder view' icon - usually at the bottom of your folder list, click on the three dots and select Folders

Scroll to the bottom and find 'Public folders'

The list of folders you have will be different, however you're looking for 'Public Folders' followed by your email or name. Either double click on the folder, or click the little triangle.
Expand 'All Public Folders:
Select the Marketing Calendar and expand that.

Right click on Marketing Shared Calendar, then select 'Add to Favorites'

To view the Public Folder Calendar alongside your own
Open 'Calendars' in Outlook:

Expand 'other calendars' and click on the box next to the calendar you want: