First download and install 'XCode Command Line Tools' either the the Apple Developer site (registration required), this is used for MacPorts later on.
Download Git, open it then right click the package and select 'Open With' then select 'Installer'. The last step is required on newer machines to stop a security warning. Install it using the default options.
Download MacPorts for your OS X version (in the section titled 'Mac OS X Package (.pkg) Installer'). As above open it, right click the installer and select 'Open With' then select 'Installer'. Again use the default install options.
Now in Applications open the 'Utilities' folder and start a terminal and enter each line below followed by the return key:
sudo port -v selfupdate
sudo port install py-wxpython-3.0
sudo port install py-serial
sudo port install py-pip
sudo port install python_select
sudo port select python python27
sudo port select --set pip pip27
pip install -U rtlsdr_scanner
Be warned it may take quite some time, but most of the software is installed now.
To complete the software installation restart your machine.
When it's restarted open 'X11' in the 'MacPorts' folder in Applications, then open a new terminal as before.
In the terminal type:
python -m rtlsdr_scanner
The RTLSDR Scanner should now be running.