For linux/ubuntu users, you need to install two packages (alternatively use the synaptic package manager)

  sudo apt-get install libopenmpi-dev
  sudo apt-get install openmpi-bin

For OS X users, install brew (after having installed xcode and gcc, needed for the gfortran compiler of openmpi) and then install with brew

   brew install openmpi

When running an executable (code.x), run as

  mpirun -n 10 ./code.x

where we indicate that we want the number of processes to be 10.