mysql_plugin: 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/mysql_plugin - from package: mysql55w-server

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

On Debian possible default full paths are:

/usr/bin/mysql_plugin - from package: mariadb-server-10.0
/usr/bin/mysql_plugin - from package: mysql-client-5.5

You may install the required package with command apt-get install {package}

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