mk-loadavg: 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/mk-loadavg - from package: maatkit

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

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