Laptop versions of graphics cards may work but are NOT officially supported.
Mac
OS
macOS Sierra 10.12.1
Processor
Intel® Core™ i5 @3.20GHz
Memory
4 GB RAM DDR3
Hard Drive
40 GB free space
Graphics
AMD Radeon™ R9 M370X (2GB VRAM)
Additional Notes
Only AMD Radeon™ graphics cards are officially supported.
Go to: http://store.steampowered.com/ and download STEAM client
Click "Install Steam" (from the upper right corner)
Install and start application, login with your Account name and Password (create one if you don't have).
Please follow these instructions to activate a new retail purchase on Steam:
Launch Steam and log into your Steam account.
Click the Games Menu.
Choose Activate a Product on Steam...
Follow the onscreen instructions to complete the process.
After successful code verification go to the "MY GAMES" tab and start downloading.