mpirun: 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/bin/mpirun - from package: mpich2
/usr/lib64/mpich2/bin/mpirun - from package: mpich2
/usr/lib/mpich2/bin/mpirun - from package: mpich2
/usr/lib64/mpich/bin/mpirun - from package: mpich
/usr/lib/mpich/bin/mpirun - from package: mpich
/usr/lib64/mvapich2-psm/bin/mpirun - from package: mvapich2-psm
/usr/lib64/mvapich2/bin/mpirun - from package: mvapich2
/usr/lib/mvapich2/bin/mpirun - from package: mvapich2
/usr/lib64/mvapich-psm/bin/mpirun - from package: mvapich-psm
/usr/lib64/mvapich/bin/mpirun - from package: mvapich
/usr/lib/mvapich/bin/mpirun - from package: mvapich

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

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