mkhybrid: 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/mkhybrid - from package: genisoimage

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

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