How to resolve "mpirun: command not found" error on Mac
You are trying to use a program but the Mac does not find the program. It's probably because it's not installed. The program you are trying to use is called Open MPI, here is how to install it: https://intothewave.wordpress.com/2011/12/27/install-open-mpi-on-mac-os-x/

After installing it you will be able to use the mpirun command. Because Open MPI isn't an Apple product, I recommend you to go to stackoverflow.com if you have questions regarding Open MPI.