You will have to register your game before you can download the updates. When you install Impulse, if you don't already, just log in with the same account you use for the forums. Then click the Impulse orb in the top left and select Register a Product. Put in your Sins serial, and the game will show up in the My Games tab and it should also find the update and flash an Updates tab (it's only visible when something you have a registration for has an update). Then just select it and hit update in the bottom left.

Strange. Make sure Impulse is allowed through your firewall(s), and try disabling Antivirus while it's patching and also run it with admin rights (right click > Run as Admin).

If it still does not work, you'll have to email support@stardock.com so they can figure out what's wrong.