adsl-start: 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/sbin/adsl-start - from package: rp-pppoe

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

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