Cities: Skylines - Airports Minimum System Requirements:
Requires a 64-bit processor and operating system
OS: Microsoft Windows XP/Vista/7/8/8.1 (64-bit)
Processor: Intel Core 2 Duo, 3.0GHz or AMD Athlon 64 X2 6400+, 3.2GHz
Memory: 4 GB RAM
Graphics: nVIDIA GeForce GTX 260, 512 MB or ATI Radeon HD 5670, 512 MB (Does not support Intel Integrated Graphics Cards)
DirectX: Version 9.0c
Network: Broadband Internet connection
Storage: 4 GB available space

Cities: Skylines - Airports Recommended System Requirements :
Requires a 64-bit processor and operating system
OS: Microsoft Windows 7/8 (64-bit)
Processor: Intel Core i5-3470, 3.20GHz or AMD FX-6300, 3.5Ghz
Memory: 6 GB RAM
Graphics: nVIDIA GeForce GTX 660, 2 GB or AMD Radeon HD 7870, 2 GB (Does not support Intel Integrated Graphics Cards)
DirectX: Version 11
Network: Broadband Internet connection
Storage: 4 GB available space

Activation on Steam

1)      Download & install the Steam client on your computer from   here.

2)      Open the Steam client, register and login. Then, locate the Games tab on the top menu. Click that and select “Activate a Product on Steam..”

       

3)      Enter your code that is showed on the orders page of eTail. Market and and follow the steps showed to activate your game.

       

4)      The game can be monitored now and you can see it on the "Library" section on Steam. You can download and install the game now.

5)      If you need more information and support, you can click here: https://help.steampowered.com/en/wizard/HelpWithSteam