ps_mem: 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/ps_mem - from package: ps_mem

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

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