If you already have PostgreSQL installed then you'll need to use the password which is already set up - try 'postgresspass' as that is another commonly used password.
FWIW if you're just starting out with PokerTracker you may want to go directly to PT4 which is currently in public Beta and freely available for anyone to check out, and much more advanced and easier to use than PT3.
PokerTracker 4 Public Beta