mmd: 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/mmd - from package: mtools

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

On Debian possible default full paths are:

/usr/bin/mmd - from package: mtools

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

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