mandelbulber: 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/mandelbulber - from package: mandelbulber

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

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