Hi there,
actually I have found some other threads dealing with this but none of them resolving the problem. This is what I get when hitting the Mesh Extract button. Sorry, I am new to ZBrush and maybe it is something very simple that I am overlooking. However, as you can see on the screenshots the mesh builds up INSIDE of the model instead on top of it.
Thanks a lot in advance 
Try to flip normals of your geometry because mesh extraction works in direction of normal vectors wich in your case maybe are facing inward.
Yup, it was my first guess too. However, flipped normals make the object look hollow, which is not the case and unfortunately does not solve the issue.
Seems like it is not my only problem with ZBrush. When I try to edit the topology of my mesh the program crashes even though I work on the lowest subdivision level, so my model has some 2500 polygons.
This is a screenshot of my preferences. I did not modify anything about it, and since I only installed ZBrush about one week ago, I do not think that reinstalling it would solve the problems.
I work on a MacBook Pro 2x2.6 GHz (Intel) with 4 GB RAM, OS 10.5.6
im having the exact same problem… have you figured out how to fix it yet?
nevermind… flipping the normals in display properties fixed it for me
unless Display > Double is enabled, rendering both sides of polygons.
v3.2, though, has the ability to extract a mesh inward. so, a simple test, set the thickness to a negative number and if it extracts to the outside of the mesh then the normals are flipped inward…
also, it looks like your mesh was created from zspheres. have you created an adaptive mesh, or are your zpheres still editable? if so, work from an adaptive skin.
oohps… this thread is old… hehehe