That's usually a problem with Impulse. They changed the login process from your alias to your email address. You'll need to update the Impulse client and log in with the email address that you originally used. The game should then be available for download.
You can ask for help in the Impulse forums, but I found it quicker to PM an Impulse employee. Be careful to note the difference between an employee and what they call a moderator there. One will help and the other has a history of berating people.
You can also go to Stardock Support and change your email address. http://www.stardock.com/support/
When you do so, you'll also need to change it with Impulse. Again, PM an employee there.
This change was brought about after Stardock sold Impulse to Gamestop.
<edit> or what Jafo said ^^^ 
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