You'll need to register the game (if not already) and use Impulse to get the latest updates. Standalone updates haven't been provided for a very long while.
In other words, get impulse off of www.impulsedriven.com, register your game and let it do the rest. After you've updated you can uninstall it if you wish.
Also this is how you should get the expansions if you decide to get them, which you may want to do as I'm not sure what the player counts are for regular Sins at the moment (though there have been a lot of new vanilla players on the forums lately).
