Try doing the ab exercises you already know while sucking in your navel. This will help you engage your lower ab muscle.
Specific exercises to try? Incline or hanging leg raises, or leg raises on the ground. When doing them on the ground, thrust your hips up at the end and push your feet towards the ceiling, pause for a second at the top and you will feel those lower abs burn. Bridges are also very effective if you remember to suck in that navel.
Google search "Transverse abdominus"