If it's a widget you're looking for, long press on an empty spot on your home screen, that should bring up options, press on widgets, find the one you're looking for long press and drag it to where you want it to be.
You can also adjust brightness in settings>display> brightness.
If you're looking to have it in your drop down menu (where you see your notifications and you can turn on WiFi ect)
Go to settings> notification panel> brightness adjustment.