Overview
In-boxes that have been auto-mapped will not always populate themselves in Outlook for Web or O365 versions. If you don't see an inbox that's been shared with you, try the following:
How to:
1. Right click your email address in the left sidebar.
2. The menu below should appear. Select "Add shared folder or mailbox"
3. Enter the email address of the inbox you are trying to access.
4. Hit "Continue". The shared inbox should now appear on your left sidebar if you have access.