mkdiskimage: 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/mkdiskimage - from package: syslinux-perl

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

On Debian possible default full paths are:

/usr/bin/mkdiskimage - from package: syslinux-utils

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

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