You will be rewarded the premium bonus for a tournament if you have been a premium member during the whole duration of the contest. You will therefore not be rewarded the premium bonus if you sign up for premium membership after the contest has started or if your premium membership ends before the contest has ended.
The bonus could be paid out in different portions, it could be an additional 10 % of your regular winnings all the way up to 125 % additional cash winnings on some occasions. You can see the pre-determined bonus percentage which is presented on the information page on every specific tournament.
You can see your added bonus winnings on the leaderboard of a finished tournament. The premium bonus is shown in a yellow text right next to your regular winnings as the picture below shows.
If you did not receive your rightful winnings make sure to submit a request with as many details as possible so that the support team has much as possible to work with in order to solve your issue.