Impulse is set on "ignore beta updates" by default. You can change that by clicking the blue button in the upper left hand corner and activiting "show pre-release versions".

Then you'll get an update notice (might require restart). Right click the game's button in the "My Games" tab and select install update.

Hope that helps

So you don't see a yellow ! icon over Sins in "My Games"? And nothing shows up if you click the Updates tab?

What does Impulse show as the available version in the box on the right?

I'm assuming you restarted Impulse a bunch of times already between then and now, but you can also try refreshing (F5), and resetting data (blue orb > preferences > folders > reset data).

Very strange. And if you click the key icon in the top right, it shows you have Sins registered?

It's supposed to show you the available update with what you've got. The only reason I can think of that it would not is if you're logged in with another e-mail that doesn't have the Sins registration attached to it.

If it does show up in your registrations though, you'll probably have to send an email to support@stardock.com so they can figure out what's happening.