You can download Impulse from here: http://www.impulsedriven.com
Install it, log in using your Stardock (forum) account, and in the top right corner there should be a button to view registrations (I believe it looks like a key - at work so I can't check).
On that screen you'll be able to add registrations, so add the Sins key and be sure to select Sins as what you're registering, since it doesn't automatically detect what the key is for.
After that, Sins should show up as "Available" when you go to the My Games tab, with the Install button in the lower right. Before you do that, though, you might want to click the blue orb in the top left and go to preferences, where you will be able to set default installation/temp files paths, and if you want it to ask you where to install.
Then, just hit the install button and it will download and install automatically. You do not need to keep Impulse running to play the game, once it downloads and installs you can leave it closed. It will also download the latest patched version automatically (v1.05), and it will allow you to re-download the game whenever you like and to archive it if you want a hard backup. Impulse will also be used exclusively to patch from 1.1 and onward.
Hope this helps 
