The abdominals are just like any other muscle group – for the abdominals to become visibly defined, they must grow larger and the fat that lies over them must decrease. What makes the visible definition of the abdominals so difficult to see is that they are situated in the area of the body that contains the most fat. Training the abdominals is only half the story.
The second part is nutrition. I can guarantee you that there is no specific diet for a flat stomach or to “burn belly-fat”. It’s all about the balance between your energy expenditure and caloric intake. If you want to lose fat, you have to take in fewer calories compared to the energy you use. Of course, you should eat healthy food (I bet that you know what that means!) and ensure sufficient protein intake, etc. In the end, you will get the results you want when you combine progressive training (resistance+endurance) and proper food.