Read this:
http://www.pokerstars.com/poker/promotions/bonus/
http://www.pokerstars.com/poker/promotions/bonus/first-time-deposit-100/
Once you’ve made your deposit(s), you earn the bonus by playing on PokerStars and collecting the number of VPPs equal to 17 times the bonus amount. The bonus is released in $10 increments, each released into your account as you clear the needed VPPs (170 VPPs for every $10 increment). You have six months after your first deposit to earn all the VPPs needed to release your bonus.