Flexion is just one function of the abdominal muscles. However, for some reason many people automatically think of flexion exercises such as situps and crunches when abs are brought up. The abdominals are used much more for stabilization than for flexion. They are also vital for any movements involving trunk rotation.
If your primary concern about working abs is due to the fair sized stomach, then you have the wrong focus anyway because working the abs is not going to do much in speeding up the loss of that stomach. Spot reduction has been repeatedly debunked and the few studies that support it show that the amount of extra attention results in extremely minimal gains as to make it a nonfactor.
For your flexion exercises, you can do crunches on the ball as previously suggested or you can do cable crunches if you can at least get down to your knees. These can be similated with resistance bands if needed.
For stabilization, I would suggest that you start out doing various planks. If you cannot get down on the floor at first to do them, you can do them with your upper body supported on a bench, aerobic steps, etc. A stable surface that allows you to adjust the height would be preferable. Start out a the height needed for you to perform the exercise. Work up to holding the planks for 60 seconds (front) and 45 seconds (side). When you do, adjust the height lower. Do this until you are doing your planks on the floor.
For the rotational aspect of the core, I like woodchoppers. If you don't have access to a cable station, you can similate these at home with resistance bands. I also love medecine ball work here. If you have access to a brick wall or other hard, undamageable surface, a circuit of side throws (both sides) and front throws will not only give you an intense ab workout, but a cardio workout as well.
I notice you said you are doing ski machine and treadmill but mentioned nothing about weights. Don't forget, almost every free weight exercise is an ab exercise as well. In fact, the best ab exercise of all just may be squats.