To make your six pack visible, the only thing that matters is lowering body fat percentage. The actual size of the abdominal muscles is a fairly small factor in how much you can see them, so the main goal is to shrink the layer of fat on top of them. However, there is no practical way to spot reduce (ie,
just target fat loss in the stomach), so this means your goal needs to be to lower body fat percentage over your entire body. To do this, I recommend the following two steps:
1. Calories in < calories out. This is the only formula that really matters with respect to weight loss (or gain) - if you eat less calories than you burn, you'll lose weight. Use
thedailyplate.com or
fitday.com to track your diet & exercise. You should aim for around 1-1.5lbs lost per week, which takes roughly a 500-750 calorie deficit.
2. Maintain muscle mass. Any sort of calorie deficit will cause your body to lose not just fat, but some muscle as well. One good way to minimize this is to keep your protein intake high: aim for ~1g of protein per pound of bodyweight per day. Another good technique is to not overdo it on the caloric deficit. Unless you are extremely overweight, losing more than ~2lbs a week will often mean that you are burning up a lot of your own muscle mass. This means that even though your weight is dropping, your body fat percentage is actually not decreasing as much as you'd like - you'll end up lighter, but still without a visible six pack. Finally, the best way to prevent muscle loss - and even gain some if you are a beginner - is to do weight training. A workout regime consisting of compound movements done with free weights (primarily the squat, deadlift, bench press, OH press, power clean, pull-ups and dips), 3 times per week, with heavy weight is a very efficient way to maintain/increase the muscle mass on your body. A routine like Starting Strength or Stronglifts 5x5 is ideal for beginners.
Where does running fit into all of this? Well, while cardio (such as running) has many benefits - including improving your stamina, heart health, etc - the only real impact it has on getting a six pack is that most forms of cardio are efficient ways to burn more calories. That is, it's only useful in boosting the "calories out" portion of the formula in #1. As for ab exercises (such as crunches), they can either increase the size (if done with high resistance, low reps) or boost the endurance (if done with less resistance, higher reps) of your abs (incidentally, running also uses your abs for stabilization). While strengthening your abs has many benefits - such as better athletic performance, injury prevention, etc - it will not significantly impact how visible your abs are.