The MacOS X distribution of Mozart uses the experimental Aqua port of Tk by Jim Ingham, which can be retrieved at http://sourceforge.net/projects/tcl. This port includes libraries from the Fink project: http://fink.sourceforge.net.
It is possible to run Mozart from the standard Emacs application provided by Apple. However, for optimal use, we recommend installing the official development port of Gnu Emacs 21 for Mac OS X (information at http://members.shaw.ca/akochoi-emacs/)