You can't buy a patch from Impulse, so I'm guessing you mean that it wants you to buy the game.  If instead you're referring to Entrenchment, that's a mini-expansion (not a patch) that does cost money.  Anyway, I'll assume you're talking about vanilla Sins and you legally purchased your game via some other outlet (Walmart, Best Buy, etc).

If that's the case, you'll have to register it to get the updates.  To do this, open up Impulse and go to the "My Games" tab.  Click on <Register Product> and enter/select the appropriate information (your email address should default to the one you've associated with your Impulse account, so don't change that).  Once that is done, your game should show up in Impulse under "My Games" and you simply highlight the game from that list and hit "update" from there.

edit - as noted above, you can also use the lightning bolt to register