On the Tools menu, click Accounts
Click Advanced
Click Delegates Tab
Under "People I am Delegate for:" Click the +
Type the mailbox name of the person, group or resource whose mailbox you want to add to your user profile.
To open additional Mailboxes click the +
Type the mailbox name of the person, group or resource whose mailbox you want to add to your user profile.
Click Find, OK, OK, Red X in the upper left to close.
Note: Relaunch Outlook. If you don't see the mailbox right away, it may take a few moments.
Problem
Unable to see additional USJ Mailbox in Outlook on a Mac
Environment
MacOS Outlook
Cause
Delegates/Additional Mailboxes need to be added manually