As is often the case, these instructions presume that you have the Xcode Tools installed, along with wget. There are instructions elsewhere on this site about how to install wget.

# make working directory
mkdir pstree
cd pstree


# fetch source, requires wget
wget http://www.thp.uni-duisburg.de/pstree/pstree.tar.gz
tar xzf pstree.tar.gz 


# build, test and install
gcc -O -o pstree pstree.c
./pstree
sudo mv pstree /usr/local/bin/