If you have access to a shared Office 365 inbox it's a straightforward step to access it.
From the File menu, choose Open ... Other Mailbox
Enter the email address or name in the box and choose it from the list. Click Ok/Open.
The new shared mailbox will now appear in the left panel. Expand it and you can see the Inbox and contents.
To send an email a the shared Inbox, open a new email as normal and there will be a new Picklist next to the From: section that will enable you to pick the email you want to send as.
If you have any questions, please get in touch with support!