mmldif: 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/mmldif - from package: 389-ds-base

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

On Debian possible default full paths are:

/usr/bin/mmldif - from package: 389-ds-base

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

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