Some people have this problem. Sometimes stuffit expander can't handle dmg files properly. Select the dmg file, go to file menu, choose Open with ... choose DiskImageMounter. Even better is suggestion below.

Or else, select dmg file, go to file menu & choose get info (or command - i) & do as this image shows & then Change All so it always chooses it: http://www.gnutellaforums.com/showpo...35&postcount=6