dmxaddscreen: 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/dmxaddscreen - from package: xorg-x11-server-Xdmx

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

On Debian possible default full paths are:

/usr/bin/dmxaddscreen - from package: xdmx-tools

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

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