ssh-add: 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/ssh-add - from package: openssh-clients

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

On Debian possible default full paths are:

/usr/bin/ssh-add - from package: openssh-client

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

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