How do you fix cracks in a tet mesh?

You fix cracks in a tet mesh the same way that you do in any other mesh, by creating couples between the two regions that are disjoined. If you created the entire model at once (you didn't import two regions and are now trying to join them) then I would check to make sure that you don't have double vertices. If you do, merge them using the vmerge command. Else, you can refer to CH 14 of the tutorial manual for instructions on how to create couples.