You'll need to download the Impulse client to access the digital DL, you can get it here:
http://www.impulsedriven.com/impulse_setup_full.exe
Then log in with the account that has Sins registered (or click the key icon in the top right to add a registration for Sins if it isn't). Once it's registered it will show up in My Games tab and if it's not installed you'll have the option to download it. If it is installed, you'll just get a play button.
Hope this helps 
