If your admin only recently added you to a shared mailbox, it may take a few minutes before the shared mailbox appears. Wait a bit and then close and restart Outlook again.
If that didn't work, then manually add the shared mailbox to Outlook:
Open Outlook.
Select the File tab on the ribbon, then select Account Settings > Account Settings.
Select the Email tab.
Make sure the correct account is highlighted, then choose Change.
Choose More Settings > Advanced > Add.
Type the shared email address, such as info@contoso.com.
Choose OK > OK.
Choose Next > Finish > Close.
Sign in to your account in Outlook on the web.
For Exchange Online mailboxes, right-click Folders in the left navigation pane, and then choose Add shared folder or mailbox. For Exchange on-premises mailboxes, right-click the name of your primary mailbox in the left navigation pane, and then choose Add shared folder or mailbox.
In the Add shared folder dialog box, type the name of the shared mailbox you are a member of.
The shared mailbox displays in your Folder list in Outlook on the web. You can expand or collapse the shared mailbox folders as you can with your primary mailbox. You also can remove the shared mailbox from your Folder list. To remove it, right-click the shared mailbox, and then click Remove shared folder.
Choose New Email.
If you don't see the From field at the top of your message, choose Options > From.
Click From in the message, and change to the shared email address. If you don't see your shared email address, choose Other email address and then type in the shared email address.
Choose OK.
Finish typing your message and then choose Send.
From now on when you want to send a message from your shared mailbox, the address will be available in your From drop down list.
Whenever you send a message from your shared mailbox, your recipients will only see the shared email address in the message.
Open the email message you want to reply to.
In the From field at the top of your message, you should see the name of your shared mailbox. If you don't, choose the From dropdown box and choose your shared mailbox.
When you want to send a message from your shared mailbox, the address will be available in your From drop down list.
Once a Shared Mailbox is added to Outlook, searching within it is similar to searching on a Primary Mailbox.
Select a folder within the Shared Mailbox on the Folder Panel on the left.
Type the query in the Search Box on top of the Ribbon (Tool Bar).
Begin the Search by pressing Enter or clicking on the Magnifier Button on the right of the Search Box.
The Message List will return results based on the Search from the selected folder on the Shared Mailbox.
After your admin has added you as a member of shared mailbox, close and then restart Outlook. The shared calendar associated with the shared mailbox is automatically added to your Calendars list.
In Outlook go to calendar view, and choose the shared mailbox.
When you enter appointments, everyone who is a member of the shared mailbox will be able to see them.
Any member of the shared mailbox can create, view, and manage appointments on the calendar, just like they would their personal appointments. Everyone who is a member of shared mailbox can see their changes to the shared calendar.