Which exercises should I be doing?

'Spot training' doesn't work very well for fat distribution. That is, if you focus on working out your abs, it'll be great for abdominal strength and muscles, but don't expect fat to disappear predominantly there. Still, if you do work on your abs, it'll still become apparent if muscle is preserved and fat is lost in general.

I've never found machines terribly great for abs. I think your best bet would be an exercise ball and doing some balancing exercises. Working your abs will require some free and dynamic movement, and I find most of the time, machines try to restrict movement. Other good exercises include (good form) push ups and planks.

Admittedly, these might be difficult at your current weight, but they will pay dividends. I still can't see any ab tone or anything, but I started at 295 back in September. Currently at 225. The size has definitely gone down significantly. If you're a particularly strong guy, you could try hanging from the gymnastic bar and raising/lowering your legs, from perpendicular to the ground to parallel.

Summary: Push ups, planks, exercise ball, suspended leg lifts.

Good luck.

I started this journey around 415 and I went to gym 5 days a week. I typically did the elliptical for 45min-hr and did the bike at times!! I did the ab machine and back exercises and leg press and arm weights.