The problem with ab workouts is that they build muscle, they don't really burn that many calories.
To burn calories what you need to do is cardio. Running, jogging, swimming, cycling, jumping jacks, aerobics and so forth. Anything where your whole body is moving for a long period of time. That burns calories fast.
Ab Workouts like the video below only come into play when you've already lost the weight and have actual abs to brag about. Then you switch from cardio to ab workouts (or balance the two) in an effort to build muscle tone.