You need a Steam Account and the Steam Client installed before using the Beta Key.
Please visit the link below to register and download client if you do not have one.
http://store.steampowered.com/
How to Activate your Steam Beta Key for Tree of Savior
1
. Launch the Steam client software and LOGIN to your Steam account.
2
. Click the Games menu then choose Activate a Product on Steam
3
. Follow instructions and enter the Beta Key in Product Activation window
4
. When the game is added to your library, press the Play button to start playing Tree of Savior.