OpenGL 2.0 compatible with 256 MB RAM, Shader Model 2.0 Support (Shared Memory is not recommended)
DirectX
Version 9.0c
Hard Drive
1700 MB available space
Sound Card
DirectX 9.0c compatible
Mac
OS
Lion (10.7)
Processor
2 GHz (Dual Core) Intel
Memory
1 GB RAM
Graphics
OpenGL 2.0 compatible with 256 MB RAM, Shader Model 2.0 Support (Shared Memory is not recommended)
Hard Drive
1800 MB available space
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.