1. You need to download and install Steam http://steampowered.com/download/SteamInstall.msi
2. Go to your account or register a new one.
3. Go to the "My Games" and select "Activate via Steam ...", and enter the key received after payment.
4. Once activated, the game will appear in the game list (Library) and you can download it
Всё прошло отлично. Ключ получил оперативно, успешно активировал.