If you have trouble on a MacOS X system, try installing version 1.9 of Aquamacs Emacs. The latest version (2.3a) seems to not work out of the box with Mozart/Oz, because it has a different name ("Aquamacs" vs. "Aquamacs Emacs"). Alternatively you can link the new version under the old name by executing the following commands from a shell:

cd /Applications/
ln -s Aquamacs.app "Aquamacs Emacs.app"
cd /Applications/Aquamacs.app/Contents/MacOS/
ln -s Aquamacs "Aquamacs Emacs"