you need to install the package if you dont see it in your winbox.
Go to http://www.mikrotik.com/download
Download the all packages file :
http://download2.mikrotik.com/routeros/ ... e-5.24.zip
Unzip the file and drag and drop the user-manager-5.24-mipsbe.npk in your router.
If your device is not running 5.24 get the user-manager for your release or upgrade your device first to 5.24
Reboot your device and the user manager will be installed.