First off, this DLC on Amazons page is listed as Mac use only, however it works perfectly with windows. All you need to do is take the 15 digit code, which you can find in your amazon account or in the email confirming your purchase, then enter it into steam. It should automatically start to download the DLC!
Now, for the game!
If you are looking at this then you are likely familiar with the Civ series and will know how addictive it is. This DLC is no different! It adds alot of new features into the game based on culture and religion, however I've noticed a huge improvement in how the AI acts. They actually move units tactically now and it adds a huge bonus to the game. They still attempt to make ridiculous trade offers but it's a step in the right direction.
The price is the main reason I went for Amazon. Steam is currently offering it for £20, which for DLC is a bit pricey for my liking, so I'd advise you to get this deal before it changes.
It's definitely worth it if you're a Civ fan and paired with Gods and Kings it makes the game utterly addictive and brilliant.