You cannot select your username because you do not have any hands/players in your database. Once you import a hand that you played you will be able to select your name.
In Full Tilt go Options->Hand History... Make sure that 'Save my Hand Histories' is checked. Make note of this directory as this is where Full Tilt is saving your hands. This is the directory that should be configured in PokerTracker.
In PokerTracker, go to your Import->Auto Import tab. If there is a check next to Full Tilt then PT3 configured it automatically. If there is no check, select Full Tilt then open the 'Full Tilt Configuration' tab on the right hand side. Check 'Automatically Start with Auto-Import'. Also, under the 'Hand History Directory', click the '...' button and browse to the directory we noted above where Full Tilt is saving your hand histories.
Now PT3 should be configured to auto-import your hands. Click 'Begin Auto-Import' while you are playing and the hands will go into PT3.

Best regards,
Derek