If you don't have the correct solid already defined, then create a cuboid by pressing the cuboid button
You must add stock objects, to tell the simulation which solid objects are to be shown at the start of simulation.In this version of HeeksCNC, only rectangular blocks are output to the simulation.
It may help to drag the Properties window to a position where all the properties can fit in it.
The cuboid can also be resized by dragging its grippers ( little circles on its edges ).
You will see the new stock object's dialog.