To access a shared mailbox, IT Services must grant your account access, with the permission of the mailbox owner.
The owner of the mailbox should complete the Shared Mailbox request form to approve access for you.
If you don't know who the mailbox owner is, you can fill the form in for yourself, however there may be a delay while IT Services tracks down the correct person to approve your request.
Once the access has been granted, you can add the mailbox by following this guide.