mpich is a freely available implementation of the MPI standard that runs on a wide variety of systems. This document assumes that mpich has already been installed; if not, you should first read Installation Guide to mpich, a Portable Implementation of MPI. For concreteness, this document assumes that the mpich implementation is installed into /usr/local/mpi and that you have added /usr/local/mpi/bin to your path. If mpich is installed somewhere else, you should make the appropriate changes.
In addition, you need to know what systems mpich has been built and installed for. You need to know the architecture and the device. The architecture indicates the kind of processor; examples are sun4 and intelnx. The device indicates how mpich performs communication between processes; examples are ch_p4 and ch_nx. The libraries and special commands for each architecture/device pair are provided in the directory /usr/local/mpi/lib/<architecture>/<device>. For example, the directory for the sun4 architecture and ch_p4 device is in /usr/local/mpi/lib/sun4/ch_p4. This directory should also be in your path (or you should use the full path for some commands; we'll indicate which).
This approach makes it easy to have mpich available for several different parallel machines. For example, you might have a workstation cluster version and a massively parallel version.