To activate and install the game you must:
1. Download and install Steam - https://store.steampowered.com/about/
2. Register a new Steam account or login into an existing
3. Go to the link received after payment and click "Accept Gift"
4. Select "Add to my library" to activate the game on the account, or "Add to my inventory Steam", to add Gift to your inventory
5. After activating the game appear in the list of games and you can download it from Steam servers
The installation (download) can take a long time and is dependent on the existing Internet channel.