Using what or which email client?
If you are using Outlook 2013 (the full application which is generally part of the Office Suite package)
Use "File" from the menu bar
Click Open & Export
Click Open Outlook Data File
Browse to your archive.pst file and select it.
You should see it added to the bottom of the left column (navigation pane)
You should be able to expand it in Outlook, find the message you want and drop it into the location (folder) in your current active outlook folders.
After finishing, simply right click the archive.pst file in the left column (navigation pane) and choose close.