04 February 2017
By default, PicaVoxel does not generate MeshColliders for voxel objects (Volumes). You must enable either a Convex or Concave collider (depending on your requirements) in the Volume inspector:
A Concave collider will give you the most accurate collision, but there are limitations with concave mesh colliders (they don't work with a Rigidbody attached).
There is also a custom collider script included with PicaVoxel that gives accurate per-voxel collison, but this has its own collision event and does not work with the regular Unity OnCollision events. Page 20 of the manual has more info, and you can also find example uses in the PicaInvaders and PicaShmup demos.