wow, 5% is CRAZY low... i think 3-5% is considered essential bodyfat. is that just from a crazy metabolism, or from the chrons? (i really don't have any idea what that is)

anyway, aside from the detailed exercises that dubcac outlined, you should start taking a weight gaining shake. it will give you the calories you need to have strong workouts, and they usually also have protein to facilitate muscle growth and creatine. and even if you doubled your body fat, you'd still be within the "athletic" range.