1. - Open Outlook

2. - Select File in the top left corner

3. - Select Account Settings 

4. - Select Account Settings again

5. - Select the "Data Files" tab

6. - Select Add

7. - If you are creating a new PST file then navigate to "Documents" --> "Outlook Files" folder and then create your PST by naming it what you would like

8. - Select OK

9. - If you are trying to add and old PST to your outlook then when you click "Add" navigate to where your PST is saved (it should be located in "Documents" --> Either the "Email" or "Outlook Files" folder unless you have saved it in a different location) 

10. - Once you click on it then select OK

11. - Select Close

    You will see it populate on the left side, you will just need to select the arrow next to it in order to see everything

NOTE: If you are creating a new PST, in order to add folders just right click on the new PST and select "New Folder"