mm3d: 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/mm3d - from package: mm3d

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

On Debian possible default full paths are:

/usr/bin/mm3d - from package: mm3d

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

Error in other languages:
mm3d: bevel nie gevind nie
mm3d: comandă negăsită
mm3d: comando não encontrado
mm3d: commande introuvable
mm3d: command not found
mm3d: käsku ei ole
mm3d: không tìm thấy lệnh
mm3d: komanda nerasta
mm3d: Kommando nicht gefunden.
mm3d: kommandot finns inte
mm3d: komut yok
mm3d: nie znaleziono polecenia
mm3d: níor aimsíodh an t-ordú
mm3d: no se encontró la orden
mm3d: no s'ha trobat l'ordre
mm3d: opdracht niet gevonden
mm3d: parancs nem található
mm3d: perintah tidak ditemukan
mm3d: príkaz nenájdený
mm3d: příkaz nenalezen
mm3d: команда не найдена
mm3d: командата не е открита
mm3d: 命令找不到
mm3d: コマンドが見つかりません