mdecrypt: 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/mdecrypt - from package: mcrypt

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

On Debian possible default full paths are:

/usr/bin/mdecrypt - from package: mcrypt

You may install the required package with command apt-get install {package}

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