To view a video ad and earn a reward, access the Side Menu, and in the "Other" section click on "Watch AD"

If the "Watch AD" button is green it means there is a video available to watch, but if the button is gray you should wait for the refresh time. You can see the waiting time above the "Watch AD" button. Keep in mind that the refresh time is 7 hours.

Every 7 hours you can watch 1 video ad to receive a reward afterward. Rewards are premium items like Mana Crystals, Mana Jewels, Mana Runes or a Booster.