mk-heartbeat: 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-heartbeat - from package: maatkit

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

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