If you have been playing with the installation, you have to be aware that if there is already a build/gnu/pref.dat it will be used unmodified, even if you give new arguments to install.com. Changes in pref.dat (either manually introduced, or by install.com if you first delete the old pref.dat) are NOT picked up by the Makefile, so unless you know exactly what you are doing you should remove all files in lib/gnu when you make changes to pref.dat before running install.com again.
Then this should work:
install.com gnu medium M MPICH
If all you do is to generate a psf it should not take one hour. Your brief message seems to indicate that you are actually reading the psf - are you having some sort of I/O problem in your system? Does this happen when you run on one CPU also?