You must claim your free spins under 'My Free Spins' on the Promotions tab. Your Free Spins will be credited automatically to your restricted balance section of your account.
If you receive free spins for a game, you will typically receive a pop up that tells you which game you can use them on. After that, all you need to do is head to the game and click the free spins icon and Bob's your Uncle! Free spins magic!