pg_md5: 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/pg_md5 - from package: postgresql-pgpool-II

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

On Debian possible default full paths are:

/usr/sbin/pg_md5 - from package: pgpool2

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

Error in other languages:
pg_md5: bevel nie gevind nie
pg_md5: comandă negăsită
pg_md5: comando não encontrado
pg_md5: commande introuvable
pg_md5: command not found
pg_md5: käsku ei ole
pg_md5: không tìm thấy lệnh
pg_md5: komanda nerasta
pg_md5: Kommando nicht gefunden.
pg_md5: kommandot finns inte
pg_md5: komut yok
pg_md5: nie znaleziono polecenia
pg_md5: níor aimsíodh an t-ordú
pg_md5: no se encontró la orden
pg_md5: no s'ha trobat l'ordre
pg_md5: opdracht niet gevonden
pg_md5: parancs nem található
pg_md5: perintah tidak ditemukan
pg_md5: príkaz nenájdený
pg_md5: příkaz nenalezen
pg_md5: команда не найдена
pg_md5: командата не е открита
pg_md5: 命令找不到
pg_md5: コマンドが見つかりません