To edit voxels, "Voxel Edit Mode" needs to be selected.

Desktop:
Click the left mouse button to add a voxel. Click the middle mouse button to remove a voxel. Hold the mouse button and move the mouse to add or remove voxels in a single plane. The plane is determined by the direction in which you are looking when first voxel is added or removed (for example, look downwards to draw in horizontal plane).

VR:
Press the trigger to add a voxel. Press the trigger while holding the grip to remove a voxel. Hold the trigger and move the controller to add or remove voxels in a single plane. The plane is determined by the direction in which the controller's ray points when the first voxel is added or removed (for example point downwards to draw in the horizontal plane). Hold both grips and move your hands together or apart to change the size of the edit sphere.