It should show up in "My Games" on Impulse Note that you should have Sins patched to 1.13 before you download it, and if you want to play vanilla Sins you should make a copy of the folder. Once Entrenchment is out of beta it'll allow you to toggle between itself and vanilla, but for now it over-writes Sins.

If it does not show up, look up the e-mail you got with the serial number and make sure that that email address matches the email addy of your Stardock account. If it doesn't, you'll have to contact support@stardock.com for an email merge.