mb: 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/mb - from package: python-mb

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

On Debian possible default full paths are:

/usr/bin/mb - from package: mrbayes

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

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