mk-index-usage: 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-index-usage - from package: maatkit

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

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