RE: Inspiron 11 3000 (3147) "Disable Touchpad when USB Mouse Present" disables touchpad ALL THE TIME
I know this is a very late reply. Most people have trouble finding it. I caught the option only by chance.
Mouse & touchpad settings --> Dell Touchpad
Click on: "Click to change DellTouchpad settings
You should be in a new window: Dell Pointing Devices.
On top you'll see two images: a touchpad and a mouse.
Click on the mouse image.
There it is! It's amazingly well hidden. Perhaps this will help someone who is searching for it.