Remember when entering the code to not skip past the number you were going to e.g if you are going to input ten and you go to eleven and then back to ten it will not be counted, I usally found Yuffie in the forest near Gongaga but I think it would be better to get her earlier in the forest by Junon. Oh yeah and make sure your pretty strong when you finally open the safe because you have to fight a boss monstor with 7000hp called lost number.