Being your first time will have a lower score than your later attempts, you should gain this naturally. Though to be safe, make sure you have a really low score the first time you play (land one parachutist). Then play normally and you will unlock this. One thing to note, is that you gain different scores on easy and medium mode. You can check your high scores in either leaderboards or statistics. Worse case scenario you can't beat your high score, you can re-set your stats in statistics menu. This would be an extremely rare situation.