cpufreq-selector: 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/cpufreq-selector - from package: gnome-applets

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

On Debian possible default full paths are:

/usr/bin/cpufreq-selector - from package: gnome-applets

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

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