DiffuseSpheres: 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/DiffuseSpheres - from package: vtk-examples

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

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