You should have been given a serial nubmer for the game in the e-mail.
Click the blue orb at the top left of the Impulse client and select "register a product" and provide the serial to get the download.
If you did not get the serial in the e-mail, you can request all your registrations be e-mailed to you via the stardock homepage, or failing that contact support. Seems a few people haven't been receiving serial e-mails the past couple of days.
