mpdhelp: 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/mpdhelp - from package: mpich2

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

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