Before you can subscribe to a public folder, please make sure you are using Legacy Outlook (checkout this link --> Which outlook version you are using (macos)
1. In Outlook
2. Go to tools in the top menu bar
3. You will have an option for public folders such as in the below picture.
4. Click on Public folder and you will see a list of folder/folders that has been shared with you.
5. Select the desired folder and click subscribe.
6. After you have subscribed to the folder it will appear on the left under "subscribed public folders"