mcdiff: 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 Debian possible default full paths are:

/usr/bin/mcdiff - from package: mc

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

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