mewstunnel: 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 Debian possible default full paths are:

/usr/bin/mewstunnel - from package: mew-beta-bin
/usr/bin/mewstunnel - from package: mew-bin

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

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