1. Copy the key you have got after transaction. You can find the key in your profile on the site Oplata.info:

2. Once you have installed the Steam client, launch the client and enter your login credentials. When you are logged into your account, click the button in the bottom left corner that says "+ ADD A GAME…" to register the product key to your account. :

ATTENTION: YOU CAN ONLY REGISTER STEAM KEYS THROUGH THE STEAM PROGRAM, NOT THE WEBSITE! THERE IS NO "+ADD A GAME..." ON THE STEAM WEBSITE ANYWHERE!

3. In the pop-up window, select the "Activate a Product on Steam" option. This will open up a new window. Follow the prompts, and paste or enter the numerical key you copied from your download page into the "Product Code" field:

4. Hit "NEXT>." If your key is valid, Steam will accept it and you will be taken to your library where you will be able to download and play your newly registered game!: