It would appear that Google have "hidden" the DEVELOPER OPTIONS menu under Jelly Bean 4.20, but it's fairly easy to un-hide it.
So, to enable DEVELOPER OPTIONS menu under JB 4.2:
1. As usual, goto Settings -> About Phone.
2. Goto “Build Number” at the end of the Scroll list.
3. Tap on “Build Number” for 7 times. While typing at the third times, you should see prompt message to indicate you that 4 more taps to go to developer options.
4. Continue 4 more taps.
5. You are now done and go back to the Setting page.
6. You should see Developer Option appear on your setting list now.
With Developer Option appeared, you can enable the USB debugging mode for future flashing and development work.
Thanks to theandroidrom.com