There could probably be a number of issues here, but to start with /usr/X11R6/bin/XFree86 has to be suid root, which it might not be if the installation defaults to a graphical login. Mine is rws--x--x, so
chmod 4711 /usr/X11R6/bin/XFree86
should do it. If that's not enough, give some more details about what error messages you're getting and/or snippets from the X log.