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

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

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