It seems a little tricky at first but basically it's fairly simple. Just one quick tip before you start...
Do NOT set the File name field to *.pst
Instead follow these simple steps:

- Specify the location of the PST files using the Look In field.
The File name field searches the Subject of the message and the Containing Text field searches the body of the email, so to display all messages leave both fields empty:

Note: To search just a specific PST file enter the full path in the Look In field, e.g.
Look In: T:\data\file Formats\Composite\pst\Sample.pst
Hit Search. You should see all the emails in your PST files showing up.
You can then start using the File name field and the Containing text field to narrow down exactly which emails you're looking for.
To ONLY search PST files in a group of folders use a Location filter, e.g.
Look In: T:\data\file Formats;+.pst
More information on PST Searching is available in the help file:
Help: Outlook PST Searching