mydns-mysql: 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/sbin/mydns-mysql - from package: mydns-mysql

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

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