Below are instructions on manually adding a mailbox to Outlook for desktop when it doesn't show up automatically. Please verify with IT that you have the appropriate permissions to view the account.
In Outlook, Go to File > Account Settings
Click on Change.
Click on More Settings.
Click on Add...
Enter the email address of the account you would like to add.
Click OK.
Click Next.
Click Done.
You should now see that account in the sidebar of Outlook.
When creating a new message, if you don't see the option to send from that account, click on From > Other Email Address... You will then have the option to send from this account.