n := elements size nthRoot: 3 ceiling
 should be 
n := (elements size nthRoot: 3) ceiling

in

on: elements
| n rowSizes rowPositions columnSizes columnPositions levelSizes levelPositions |
n := elements size nthRoot: 3 ceiling.
"Initialize the cell sizes"
rowSizes := Array new: n.
columnSizes := Array new: n.
levelSizes := Array new: n.

Otherwise, Array is initialized on a float and that gives an error.

usman