Every day someone tells me that the problem with Pilates for them is that they can't find their abdominal muscles, either from disuse, abdominal surgery, or other stress. Short of having radar detectors to zero in on this area, focus and attention are the next best thing.
Start with breathing. Your abs assist the movement of your ribcage and therefore your diaphragm to help you breathe. And these muscles are not just in the front, but actually wrap around your torso back to front, forming and actual internal corset of strength and support.
So when you inhale let your ribcage expand in all directions and feel your abs relax. As you exhale feel your ribs and abs pull in towards your waistline. That movement is being done with the help of your abs - you have found them!