d.mon: 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/lib64/grass-6.4.4/bin/d.mon - from package: grass

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

On Debian possible default full paths are:

/usr/lib/grass64/bin/d.mon - from package: grass-core

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

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