To show the On My Computer folders in the navigation pane, on the Outlook menu, click Preferences. Under Personal Settings, click General, and then, under Sidebar, clear the Hide On My Computer folders check box.



Source: https://support.office.com/en-us/article/about-on-my-computer-folders-in-outlook-for-mac-c91b8729-924d-4c25-a5f6-38883d0f763d