Personal Archive can be found as a subfolder under your Inbox in Outlook.
When you click on the folder, wait a few seconds for the page to load as it is loading a webpage within Outlook.
Note: All emails sent and received to your mailbox are automatically archived in to Personal Archive and are kept permanently. You do not need to manually archive emails in to Personal Archive, a copied is placed in Personal Archive as soon as it hits the email server - that is before you see it in your Outlook.