If your BMI is 11 you should be dead.

It's ok though, it's extremely unreliable as an indicator of body composition. Muscle weighs more than fat, so anyone who's remotely in shape and has a decent muscular frame will be well over 23, ie "overweight".

Abs are mostly about cutting down body fat percentage, like Dale said, to below 10%. To to this, hit a low carb diet without cutting back on proteins, and try to increase your intake of good fats (mono- and polyunsaturates, like you get in olive oil). Keep hitting crunches and planks, but I'm afraid it's a lot of grilled chicken breast and steamed veg for you m'lad.