You should read more about arrays first: http://rathena.org/wiki/Variables#Array_Variables
The problem should be clear:
.RewardList[.Reward]
Where .Reward is never defined, defaulting to "0". To loop through an entire array, you'd use, well, a loop:
for(set .@i,0; .@i<getarraysize(.RewardList); set .@i,.@i+1)
getitem .RewardList[.@i],.RewardAmount[.@i];