meld: 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/meld - from package: meld

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

On Debian possible default full paths are:

/usr/bin/meld - from package: meld

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

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