umbrello: 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/umbrello - from package: kdesdk

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

On Debian possible default full paths are:

/usr/bin/umbrello - from package: umbrello

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

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