The main difficulty in installing MRtrix on MacOS X is to install the dependencies. There are three reported ways to do this, MacPorts, Homebrew, and Fink. At this point, it seems MacPorts is the simplest and quickest method, as it only requires a minimal amount of local compilation - most of the required packages are simply downloaded ready-compiled.

These approaches are described below, along with instructions common to all of them.

Installing dependencies

The first step is to install Xcode and X11. These can both be found on the OS X install DVD. The MRtrix-specific dependencies can then be installed using one of the following three approaches:

MacPorts (recommended)

Thanks to Craig Stark for providing these instructions.

Install MacPorts
follow instructions on http://www.macports.org/install.php.
Get and build the dependencies
$ port install glib2 glibmm libpng gtk2 gtkmm gtkglext gsl pkgconfig

Homebrew

Thanks to Jon Clayden for providing these instructions.

Install Homebrew
follow instructions on https://github.com/mxcl/homebrew/wiki/installation.
Get and build the dependencies
$ brew install python glib gtk+ glibmm gtkmm gtkglext gsl

Fink

Thanks to Michael Zeineh for providing these instructions.

Install Fink
follow instructions on http://www.finkproject.org/download/
Install the dependencies
$ fink install python glib gtk+ glibmm2.4 gtkmm2.4 gtkglext1 gsl
If this fails, try to repeat this last step until it works.

Compile MRtrix

At this point, you should be able to compile MRtrix using the following commands, as per the Unix installation instructions:

$ cd mrtrix-0.2.10
$ ./build
If this completes successfully, you can now install the executables into a more easily accessible location. This can be done using the following command (as root):
# ./build install

Final configuration

Assuming you have managed to install MRtrix correctly, you should also consider setting a few configurations parameters, as per the Unix instructions.