From : https://www.scirra.com/manual/108/array
Set at XY
Write a value at a position in the array. Indices are zero-based. Writing to values outside the array has no effect. If Set at X is used, the Y and Z indices are 0. If Set at XY is used, the Z index is 0.
So for ball
Set at x = 0, y =0, value = "ball"
Set at x= 0, y= 1, value = 10