The pam script by default launches MPI runs with mpirun. If you cannot use mpirun because you are on a system which does not support it or which offers alternative MPI launchers (SGI, Cray), you can define your own DIRAC_MPI_COMMAND like this:
$ export DIRAC_MPI_COMMAND="mpirun -np 8"
$ export DIRAC_MPI_COMMAND="mpprun"
$ export DIRAC_MPI_COMMAND="aprun -n 24"
This will then launch:
$DIRAC_MPI_COMMAND dirac.x
instead of:
mpirun -np N dirac.x
Note that --mpi overrides DIRAC_MPI_COMMAND.