Unfortunately today the extension database did not work
Every day the Slicer Preview Release shows up around 1am EST but extension builds are completed around 10am EST, so depending on your timezone, you may not see extensions for a while. You can retry installing extensions now or install yesterday’s release (Download 3D Slicer | 3D Slicer).
The holes you see in the mesh are not there because you have such big holes in the bone. It is a segmentation error, because the average intensity of the bone decreases due to partial volume effect and it becomes so similar to soft tissue’s intensity that they become classified as soft tissue = outside segmentation.
Therefore, you need to fill in these “holes” in the segment and to reproduce material properties, you set different material properties within this mesh. You can take samples of the CT volume at every mesh point to decide where in the mesh you set material properties corresponding to cortical/cancellous bone.