dsniff: 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/sbin/dsniff - from package: dsniff

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

On Debian possible default full paths are:

/usr/sbin/dsniff - from package: dsniff

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

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