var MyAtrNoSma = new double[barsCount];
for (int i = 0; (i < barsCount); i++)
{
MyAtrNoSma[i] = (source.HighPrices[i]-source.LowPrices[i])/source.LowPrice[i];
}
var MyATR=Series.SMA(MyAtrNoSma, Per);

и убрать строку
double MyATR =0;