Solution 1 - Exit the game and re-open it
If you don't see the game window as the game launches automatically and it is not the full version, then you can in most cases exit the game and then upon re-opening the game the required game window will show.
Solution 2 - Uninstall and redownload the game again
1. Uninstall the game from your computer
2. Redownload the game again through the game description page by choosing the Play Now option in green
3. Once the game is installed, the game window should now show so you can unlock your game
4. If the game launches automatically, exit the game and the game window will show now and you can then unlock your purchased game.