When you purchase the game you will also receive an activation key. The activation key is usually on a white sticker on top of the CD, or inside the front cover. The activation key is in form of XXXX-XXXX-XXXX-XXXX.
PC games can be found in various retail locations and online at Amazon.com for example. However, please note that these games will not receive further updates and may not contain all features present in the mobile versions.