Zim is an amazing app (see my demo). Unfortunately, there is not yet a simple click-and-install option. Instead, installation requires use of the terminal.

Homebrew method

Download and install Xcode from App Store (optional: minimal install)
open terminal and install homebrew package manager by pasting the following

ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

install necessary brews and then make sure your path to homebrew's python is correct.

brew install gtk-mac-integration python pygtk gtkspell3 wget

install Zim
1. get source of latest Zim

mkdir ~/bin; pushd ~/bin;
wget http://zim-wiki.org/downloads/zim-0.67.tar.gz
tar xvfz zim-0.67.tar.gz
rm zim-0.67.tar.gz; popd

install dependencies

/usr/local/bin/pip2 install pyenchant 
/usr/local/bin/pip2 install pyxdg
/usr/local/bin/pip2 install pygtkspellcheck

test Zim

/usr/local/bin/python2 ~/bin/zim-0.67/zim.py

optional: customize Zim
1. create icon (make sure you use brew's python in /usr/local/bin)
2. set your preferred browser (for me it's Chrome)
  1. alt-click on a link and set browser to

/usr/bin/open -a "/Applications/Google Chrome.app"

Zim on Mac OSX

Homebrew method