When you register with a casino that is offering a free spin bonus, the bonus is normally added to your account once you've completed the registration process. In some cases, there will be a message when you first login to the casino account that will ask if you want to take advantage of the free spin bonus. If you choose to play it, then the casino software will automatically take you to the game that is already setup for the free spin bonus. If you login to the casino and there is no mention of a free spin bonus, then go directly to the featured slot game that is mentioned as a part of the bonus, and the free spin feature may automatically begin. This is how free spins are normally activated.
If neither of these options work for you, then it is best to contact the casino support and let them know that you signed up for a free spin bonus on a specific video slot game, but it hasn't been credited to your account. In most cases, they will add the free spin bonus, and request that you go to the video slot game. Make sure that you are specific about the bonus, because they may have multiple bonuses running at the time.