PokerStars doesn't write the placement information into the SnG histories. Because of this PT3 can't read the finish positions while playing.
However, you can request the tournament summaries to be e-mailed to you to be able to import the results. To do this, open your PokerStars client and go to Requests -> Tournament History. After you have filled in that box, you will recieve an e-mail. Copy this e-mail into an empty text file and save it, then import it with PokerTracker.
Eventually, PT3 will have a feature to make this more convenient so you can set it to read from your e-mail.
Also, we do have some documentation that can help getting you started located
here.