Setting the cube is quite simple while you're in edit mode. Simply click the cube in the board and the cube selection widget appears.
Selecting a cube in the first row, where the number is displayed up-side-down, the top player will be the cube owner. The value of the cube will be the value of the cube you click in the widget. Note that the unturned cube is the leftmost cube labeled 64, just as it usually is on real double cube.