It is straightforward installing GraphViz for Mac OS X. The easiest way might be using the Mac port of the same name, GraphViz. However, I think this has not been updated in a couple of years.
You can install the commandline version of the latest GraphViz tools, dot, neato, etc by using MacPorts. Assuming you have MacPorts properly installed and configured, I think the following command is used(I have to check to be 100% sure):
sudo port install graphviz
Once the installation is done, do a simple sanity check by typing in
neato in the Terminal.