ref: http://www.ninjaromeo.com/enable-usb-debugging-developer-options-jelly-bean/
How to Enable USB Debugging and Developer Options on Android 4.2 Jelly Bean:
- Go to – Settings > About phone.
- Then move on to Build number option
- Tap on Build number repeatedly about 7 times.
- This is enough.
- After touching 7th time you will see a message that, “you are now a developer”.
完成七次之後,回到上一層,就會看到多了一個開發人員模式了~~