mpirun_rsh: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/lib64/mvapich2-psm/bin/mpirun_rsh - from package: mvapich2-psm
/usr/lib64/mvapich2/bin/mpirun_rsh - from package: mvapich2
/usr/lib/mvapich2/bin/mpirun_rsh - from package: mvapich2
/usr/lib64/mvapich-psm/bin/mpirun_rsh - from package: mvapich-psm
/usr/lib64/mvapich/bin/mpirun_rsh - from package: mvapich
/usr/lib/mvapich/bin/mpirun_rsh - from package: mvapich

You may install the required package with command yum install {package}

Error in other languages:
mpirun_rsh: bevel nie gevind nie
mpirun_rsh: comandă negăsită
mpirun_rsh: comando não encontrado
mpirun_rsh: commande introuvable
mpirun_rsh: command not found
mpirun_rsh: käsku ei ole
mpirun_rsh: không tìm thấy lệnh
mpirun_rsh: komanda nerasta
mpirun_rsh: Kommando nicht gefunden.
mpirun_rsh: kommandot finns inte
mpirun_rsh: komut yok
mpirun_rsh: nie znaleziono polecenia
mpirun_rsh: níor aimsíodh an t-ordú
mpirun_rsh: no se encontró la orden
mpirun_rsh: no s'ha trobat l'ordre
mpirun_rsh: opdracht niet gevonden
mpirun_rsh: parancs nem található
mpirun_rsh: perintah tidak ditemukan
mpirun_rsh: príkaz nenájdený
mpirun_rsh: příkaz nenalezen
mpirun_rsh: команда не найдена
mpirun_rsh: командата не е открита
mpirun_rsh: 命令找不到
mpirun_rsh: コマンドが見つかりません