How to open a MS Outlook mail file backup PST file from within Outlook: (based on Outlook 2013)

Open Outlook and click on "File" (see 1 below)

Click on open & export (see 2 below)

Click on open Outlook data file (see 3 below)

Browse to the location, select, and open the file (not shown)

The Outlook PST file folder structure should, now appear below your normal inbox (see 4 below)

Its a good aide to close the file afterward if you won't be using all the time to do that right click on the folder structure and chose "Close Outlook data file"