mk-visual-explain: 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-visual-explain - from package: maatkit

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

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