pydot.InvocationException: GraphViz's executables not found

I try to run this example for decision tree learning, but get the following error message:

File "coco.py", line 18, in graph.write_pdf("iris.pdf") File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pydot.py", line 1602, in lambda path, f=frmt, prog=self.prog : self.write(path, format=f, prog=prog)) File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pydot.py", line 1696, in write dot_fd.write(self.create(prog, format)) File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pydot.py", line 1727, in create 'GraphViz's executables not found' ) pydot.InvocationException: GraphViz's executables not found

I saw this post about a similar error, but even when I follow their solution (uninstall and then reinstall graphviz and pydot in the opposite order) the problem continues... I'm using MacOS (Yosemite).

Any ideas? Would appreciate the help.