Im pretty skinny but i want to be skinnier so my abbs show?

hello im 15 and im pretty skinny id say. but i really want a 6 pack its been my goal for awhile now. ive donw p90x and insanity, but i never did the whole thing i only did the abb routines for them. my BMI is 18, which is just over underweight i suppose, but i heard you have to have a bmi of 12 to have your abbs really show. i way around 115 i believe maybe a pound less, and im 5"6 maybe slightly taller. if i lay down and there is really shadowy lighting then i can kinda see a 4 pack, but when i stand up it disapears into my stomach. i want to get it so it really shows well when i stand up. i do things like crunches situps, abb workouts that i got from p90x, and lately ive been doing stuff with a 10 lb medicine ball because abbs are like a muscle the more you weight you use the bigger they get. i have protein shakes and protein bars i usually do to enhance my workout. but i guess wht im trying to say is how should i lower my bmi? no i am not anorexic or trolling or whtever.i just want it to show better and i do eat alot but its mostly all healthy stuff. im also a male if this helps. my personal trainer at golds gym said i was just right to get a 6pack but it doesnt come out no matter how much i do. if i pull back my skin very tightly i can see a pretty indented 6pack but when i let go you cant really see it. so should i lower my bmi? if so, how? and or should i just keep working my aba until they pop over my skin more?